So you're saying that I can buy a game with Steam, remove Steam from my computer, and then run the game I bought on Steam? No, afraid not... at least not the last time I tried, anyway. To be listed on Steam, you need to add hooks into your software that interface with the Steam launcher, and that keep the game from running (unless you hack it). As far as F2P games, I'd rather not use Steam as they still take 30%.... At least Epic is only taking something like 12% off the top.
The Twitch Prime games seem to run without needing the Twitch desktop client installed, but their listings, for the most part, have been mediocre. I think I've gotten a couple of decent games this route, but not enough service for just Twitch. At least it runs concurrently and included with my Amazon services. (Now if you want to talk "ripping you off while giving you free/cheap shit," there's a company to be wary of... along with Facebook and their ill-conceived ilk.)
Oh, meant to say... I'm up for some Civ. Seem to be having network connection issues with their (2K) services, though. Also, the "Red Death" format worth trying out? Seem to be some mixed views on it. Just had to get my rant out first.

Opinions expressed in this statement are the authors alone and in no way reflect on the game development values of the actual developers.