installers for games are usually just a script that unzips the game and makes some shortcuts. Steam installs all your games in a standard way in a folder of your choice. You can straight up copy that folder to another computer. You can use another launcher and just play your games, there are already many that can read steam's standardized format. I've done it multiple times to avoid redownloading my library
It depends how steam sunsets their DRM, but yes - obviously if a game has 3rd party DRM, that third party is in control. Steam could choose a user hostile way to sunset their own DRM, but they could release ways to deactivate it

DRM is bad, steam provides an easy way for developers to use steam DRM, and it's generally less user hostile than most DRM. To me, this seems like harm reduction

Ultimately, it's not up to steam what, if any, DRM a game uses. They manage their in house offering, but the developer doesn't have to use it if they don't want to