1. Regular Win32 software support is probably never ever going away, because it's the bedrock of the Windows platform to this day.
2. Universal Windows Platform apps can be installed by third parties just like normal applications. Steam could support distribution of UWP apps if it chooses to.
3. Cheapy Chrome OS alternatives aren't going to be competing with gaming PCs that are the bedrock of Steam... ever.
Tim's pretty much flat-out refused to recognize these facts, and has been scare-mongering this point since Windows 10 came out or before.
UWP being a sandboxed app platform is just "better" than the legacy software design from a security standpoint. Nothing about UWP inherently makes it lock out third party distributors, it's just that nobody else has chosen to distribute them yet.