Linux and running games under steam/wine/proton is great in the broad strokes, but users will have built up their own collection of tools or ways of doing things they will seek out equivalents for and judge the linux experience as a whole on whether they can do that. Many of the windows applications are very mature compared to linux because that's the ecosystem and audience its had for decades, there's nothing touching Foobar2000 for example (and the UI glitches in wine). Now add in all the other things gamers regularly expect to do, what's needed to accomplish them and how well they do it, overlays, screen recording, using modding tools, etc.
It also strikes me with the win10 end of life there's going to be a huge variety of hardware configurations people want to 'just work', in terms of age and which model someone chose in a particular generation. For example support for fan control on my Z270 board doesn't exist, presumably because of the way ASUS made that model.
I can appreciate Valve and their direct partners picking their battles on what to support as it's a huge gauntlet to pick up, but I really doubt the needle is going to move large distances and saying "bye bye windows gaming"