Guilliman is a Mary Sue of the highest order on top of that though and Ultramarines have been writers' pets for ages.Space Marines are the John Cena of 40k.
And by that I mean a specific thing. I believe most fandoms have a John Cena. An element to the game that is especially marketable, but to a wider audience that is less invested. The people really involved get tired of the popular, profitable character being pushed all the time, not because they hate that character, but because they are so invested in the media that they get tired of it, because it is repeated very frequently to them.
Broly is Dragon Ball's John Cena. Jace is Magic the Gathering's John Cena. Space Marines are 40k's.
I know all don't match my description (case could be made Goku is DB's John Cena), but that's what I call it when there is a quiet majority that likes a character, that they very invested fandom loathes for it's over usage.