Valve/Steam does not require games distributed through its platform to use their SDK/DRM in any way. It's up to the individual game developers if they set such a Steam a trap.

Valve doesn't even give an outright monetary incentive to do it. i.e. Valve's 30% cut doesn't turn into 10% if you link in Steamworks.

Though one could argue there's an indirect monetary incentive created by locking away all the platform/social features behind a wall accessed through their proprietary library. But last I checked the DRM aspects of that library could be ignored while using the rest of the platform. It does automatically stay updated though, so they could turn Evil and force DRM @ initialization if they wanted, hypothetically speaking.

It depends. Some games are DRM free, and those can be backed up and kept wherever you wish. I can copy my Kerbal Space Program directory anywhere, for example, and it works fine. I use this to have several differently modded installs.

You can absolutely retain what you've bought: all content is saved in the steam/steamapps/common folder. If you want you can rip those files out and run them separately with cracks or Steam emulators like Goldberg, but the supporting infrastructure and dependencies for games (like multiplayer services, matchmaking, official support) will be lost. Many games prefer to rely on Steam's platform and support for core features by their own design.

There are DRM-free games on steam, but how many people maintain backups of their games? And this is not limited to steam. Most distribution these days on PC is digital, and even consoles moved there. If you lose your account for whatever reason, you will lose them all. Same problem with other content (video, audio, etc.). As long as there is no law protecting the customers from those things, it will remain a problem.

What you're saying is absolutely true, not sure why people are trying to bury your comment.

Not if you want to use your software outside of steam, no.

Yes, it's like netflix/etc in that regard.

Steam is not a subscription service so I'm not sure what you mean by "leave".