especially when steam could legit actually tell publishers they can’t kill their games when it’s sold through their platform.

Steam actually does take a hand in this to a degree. For instance, they don't apply any mechanisms wherein a player who bought a game simply "cannot download it" anymore. They can't do anything to force online servers to stay up, but for example: You can't buy the Telltale Law and Order game anymore. I can still download it to my Steam account on any device, because I bought it when it was around.

Steam definitely did not popularize DRM - there's a reason GabeN is attributed for the "piracy is a service problem" quote. He made Steam because the most common DRM systems of the time were a huge roadblock for gamers. In turn, he created a system that was appealing for both players and publishers; even if it's not as consumer-open as GOG.