If I buy a game I expect to have access to it forever, not for as long as an account or someone else's servers are active. Steam and other DRM services do nothing to stop piracy and being satisfied with my product does not come second to anti-theft measures. Attitudes like yours are what has caused DRM to become such a (rightfully named) stigma to the PC gaming landscape. You are not the developer, not the one I'm asking and nobody had you to come and try to convince me to not even ask my question. Please do not reply again.
"have you ever seen a game that's been published on Steam and also on another site?"
All the time? Humble store, GOG and even this very website itch.io, which by itself has Brigador, Nuclear Throne, Snakebird, Captain Forever Remix, Sky Rogue and hundreds if not thousands of others which are all also on steam. In fact, many games on the humble store have both direct downloads and steam keys available. Look it up sometime.