> Getting your Steam library to work on Linux before it got Valve's blessing with Proton wasn't a great experience.

There weren't any real roadblocks for that caused by Valve. And it definitely wasn't as hard as you're implying.

> If they wanted to, they could have easily decided to block games from running on Linux and gave some statement about preventing piracy and protecting users from malware.

They could have just like any software developer could but they didn't. They also didn't block the Steam for Linux client from running on unapproved distributions or even FreeBSD.