The odds of Steam/PSN/XBL shutting down or removing paid content from an account are vastly smaller than the odds of a house fire, robbery, or other physical event claiming your precious vidya collection. It's basically the difference between storing your money in a bank and cramming it under a mattress.
The fact that you can only find one extreme example (a free promotional teaser for a cancelled game) of a reputable (i.e. non-Nintendo) digital service removing a claimed product from user accounts tells me all I need to know about the long-term reliability of digital vg content.
There are a slew of games that have been for sale on iOS, and have since been fully removed and rendered unplayable. As in, the game can't be redownloaded, and even keeping it on your device doesn't guarantee it'll be playable in the future due to future versions of iOS being incompatible, or even a simple failure to call home. The Binding of Isaac, Bioshock, and the three Shadowrun Returns games are all examples of well-known games to completely get the axe, and all were around $15 each.
Same goes for the Google Play store, Shadowrun Returns is gone from there too, though Android is way more flexible and can still run a lot of older games on newer firmwares.
There's also a case with how games, even on major platforms, can get updates that remove content, and I think the most notorious instance of that was Grand Theft Auto 4 losing some radio songs due to licenses lapsing. Of course, the work around is, just play it off the disc without updating.
Then again, the real trouble these days comes from games that call home, to where you need to be online to even play in single player. Like Battleborn, which is set to be completely unplayable by the end of January 2021, even if you have the disc. And if you want to laugh at spergs, Lawbreakers is already in that boat, and even had a physical copy printed and released on Limited Run Games. Let's see how much that goes for on eBay:

Ah, yes, $120 for a mint condition copy of a game that is entirely unplayable. Money well spent.