Keeping users on the Microsoft platform is part of the advantage of having their own console environment. The Xbox OS bears a lot of similarities to something like Windows 10 S in that regard, except obviously geared towards gaming rather than basic computing.
The Steam rumours are certainly interesting. It would no doubt have to be a curated, modified version if it ever comes to fruition. Releasing full PC Steam into the Xbox environment would no doubt be chaotic.
Still, as someone who has a fairly significant Steam library, it is an interesting idea.