Steam isn't paying developers to keep their games off gog, or epic, or origin. Calling steam a walled garden is ridicolous.
You're free to buy the games on other stores, you just won't get acess to the steam workshop (Well not officially), I think comparing this to Epic money hatting games is pretty disingenuous.
If epic had a feature that steam didn't have and you only got access to it if you used their platform that would be more than fine.
It's just as bad when consoles do it, but console users have been used to that shit for decades, it doesn't make it good, it's just the norm. I'd like it not to be the norm on PC too.