(Special thanks to @The Gangster Computer for the following):
Alan Horn, Kathleen Kennedy, Bob Chapek and Bob Iger were instrumental to the decline of Star Wars as a major franchise. Each one contributed a unique brand of self-destructive egoism and conflicting ideals that resulted in a metaphorical nuclear meltdown that started with a bright flash that was mistaken for awe-inspiring light but was really just the blast before the shock wave.
Alan Horn (chairman of WD studios and later creative officer) was all about deconstructing the franchise and remaking it into something completely unrecognizable because he supported the idea that Star Wars should focus on appealing to those who didn't care about Star Wars and whose decisions were what you would expect from an out of touch ivory tower boomer.
He was essentially a guy who had never delved into the franchise or understood anything about George's mindset or mythos. One of his earliest acts was to question everything by claiming stuff like "What is the Force?" "What are the Jedi?" and following it up by somehow rationalizing it that it was their turn to finally explain everything and really create a non-existent conclusion to a franchise that had already given more than enough closure and fanservice material. He and Kennedy also orchestrated (presumably with Iger or Chapek's blessing) the downsizing of Lucasfilm and the complete replacement of most of its old staff with fart-huffing "new age creatives", even getting rid of many Lucasfilm veterans and only keeping a few of the concept artists who had been working in the studio since the prequels, as well as a few infamous ass-kissers like Pablo Hidalgo and Matt Martin.
Alan Horn effectively replaced most of the staff with Hollywood pr people or fresh-out-of-college "visionaries" who had no prior experience with Star Wars or even sci-fi, fantasy or space operas (or even writing) while completely undermining everything George and his writers had ever set up. And there's the achievement of the "Creative" officer. Alan Horn and Kennedy adherence to his ideas ultimately resulted in further conflicting directions which resulted in the disjointed narrative that has been following Disney "Canon" since its inception. There's more on him but you can just use search in this thread only and find more tidbits on his failings. Overall, I consider him and Iger to have been the most detrimental elements, with Kennedy as a close second behind them.
Pablo Hidalgo and Matt "Porgcuck" Martin (story group and former website managers that were needlessly promoted) in their own way also contributed to the division in the fandom by sowing seeds of conflict and making up reasons for the fanbase to fight via twitter and on forums as well as looking for reasons to claim why everything from George's era was dumb, or even making false claims about the state of the franchise which they even backtracked on a few times, as well as betraying old fellow employees when they demanded royalties or misleading them by claiming that they would still have their media published in Lucasfilm.
Leland Chee was another asskisser but who contributed less to the decline and division than Martin and Hidalgo, and acted more as a doormat who no one listens to and doesn't dare go against the grain, with his only purpose being to seemingly try and calm the flames the other two kept feeding, but he even failed at that, much like most of his tasks that required more assertiveness. His and Hidalgo's lack of assertion and competence were also the main reasons for so many continuity errors in Disney and the final days of old Lucasfilm.
Kennedy and Horn also served as the greatest roadblocks for creativity and adherence to George-era star wars, even meddling continuously in the productions of the new films, most especially their spinoffs, resulting in not only unnecessary budget costs but in large amounts of footage, sets, props, costumes, scripts and even directors constantly being replaced on petty whims to make sure everything adhered to some moronic "new vision" of Star Wars.
Overall Kennedy's role in the company has been a relatively useless one that has been mostly self-serving and adhering to the whims of men rather than her own, and her actual decisions have all been flabbergasting or ignorant, even dismissing classic costumes from past films as "not being like Star Wars" and having a bizarre obsession with tentacles and sea monsters, and even hiring just as many incompetents as Horn, most of which were only hired to appeal to investors who wanted a more "progressive" image for Lucasfilm and were made up of soap opera writers for the story group or more inexperienced college interns. It was all a shameless ruse that took advantage of shameless consumers, the politically obsessed and brainless investors who can't tell when they're being manipulated, all in a bid to obtain more money and influence for the greater company, the whole supposed 'feminist' angle was also a method to attract reactionaries against them in order to play the victim card and make their faux feminism look genuine and sentimental, which was at first exercised through cringe tweets by JJ claiming all criticism was racism or sexism and Lucasfilm's bragging about Phasma's armor to attract GamerGate medias which at the time were also in the midst of a feud regarding female armor in games. It was all a cleverly orchestrated market strategy based around a faux illusion of altruism to increase publicity and it resulted in the birth of the uselessly idiotic Fandom Menace youtube movement which only helps to boost interest in Disney rather than discourage it, with noteworthy examples like Doomcock and Geeks&Gamers who seemingly criticize Disney at every turn yet encourage their viewers to continue watching and buying Disney Wars material, basically making them more crypto shills who are too stupid to realize they're being used for publicity with their useless takes.
Bob & Bob
He brought about the largest economic growth for the Walt Disney company by turning it into an even bigger corporate monster hellbent on controlling everything around it. This growth proved to have little benefits for the employees within the company, as even Abigail Disney (an actual disney heir) has pointed out Iger's corruption regularly, his and his executives habit of giving themselves raises on a regular basis while denying their employees even basic necessities and taking advantage of others willingness to work for the company, as well as how their substantial greed has resulted in a lack of money going to areas of the park and company that really need it.
It was also his decision that the original treatments for George's sequels be completely ignored, preferring to target a new demographic, mainly those who had no interest in Star Wars and who fittingly enough moved on from it as soon as the sequels disappeared, with the only ones sticking around being old school addicts and your typical disney fanboys who will lap up whatever has the disney logo on it.
Bob Chapek (Chairman of Experiences, Parks & Products as well as Iger and Everyone's primary buttkisser) was the royal cheapskate of Disney and the one not only responsible for the unimaginative blunders that were the star wars park expansions and hotel, but was also the one who was most fixated on taking the cheapest routes for max profits. Since taking the CEO position, his current management of the Star Wars brand has also been one of cheapest alternatives for max profits resulting in even lower quality blunders than usual.
This budget issue became a more serious problem when George grew interested in making a new series of films, something Lucasfilm at the time (as well as the costly budget of a certain show among other things mentioned) just couldn't do, resulting in George ultimately trusting Kathy and Iger's suggestions to sell Lucasfilm to Disney in hopes that their large assets and connections would allow for the new films and new media to continue unhindered with the hope that due to their long standing "friendship and trust" they would honor his work and continue his vision, aaaaaand you know the rest. George's involvement to this day is mostly for public image and supposedly due to some sort of agreement which keeps him from actively criticizing Disney, with the few instances that he has having been backtracked or erased, with the exception of a few and some production visits for the Disney+ shows which really don't amount to anything beyond cameos.
Ultimately, this can all be traced back to around 2010, but 2011 is where you could already see many changes start to occur presumably to accommodate or appeal to Disney whose deal would be made official in 2012, such as an abrupt number of cancellations, the complete overhaul of the official SW.com website, the Lucasfilm website also receiving a complete overhaul where it eventually resemble a shopping network website, the databank being deleted (despite containing a plethora of concept art, production info and setting lore), the disappearance of the EU and Infinities labels (Canon would be introduced around 2014 iirc), the sudden removal of many non-Filoni Wars merchandise from lists (as well as the cancellation of Filoni Wars which was finalized in 2013 due to contractual disputes with FOX and CN since Disney wanted full ownership of all SW media going forward), the disappearance of many recognizable labels, many producers, employees and authors suddenly going silent and most notably the shut down of the official Star Wars message boards, a massive loss since it was a place where Lucasfilm employees, Star Wars writers, authors, etc and fans all came together and expanded on the mythos and even contained info on unreleased media and background information on the films, George's notes and other productions, including info and images that otherwise went unreleased in any official magazines or guides, and which too many morons were too lazy to archive and because many didn't know about archive.org or the site's eventual demise (this was a forum that had been active for almost 10 years since 2001).
The only thing that is presumably still under George's influence (although the full extent is uncertain) is the original versions of the OT which Disney has not yet released (although George had permitted their screenings at film festivals completely unaltered), including newer edits being made to them such as the infamous 'maklunkee', but again the full details of this are unknown.