well, many games are tied to the steam client (through the steam runtimes, steam DRM, steam input, needing a steam account for online play...). for most games, no, you can't just take the executable and do whatever you want with it. you'll need the steam client, and this creates a lock-in effect. because you need steam open to play all your steam games, you won't look elsewhere for games, and you won't see games not on steam, unless they're big enough.
imo, the solution to this is to break the lock-in, have interoperability between clients. there's no good reason why cross-play between steam and GOG, for example, is an exception and not the norm. there's no good reason why the steam client is required for so many games, there should be offline installers. there's no good reason why steam input only works with the steam client. part of the reason why proton is so amazing is that it's open-source, other steam technologies should be the same!