> Because that is ultimately what makes Windows games run on Linux, and represents Valve's failure to change the culture of game studios to consider GNU/Linux worth targeting
Unfortunately, eliminating the diversity of the Linux ecosystem would also be eliminating one of its strengths. Proton covers up that diversity, making it easier for game developers to target Linux (whether they do so intentionally or incidentally). While a few Linux gamers may be bothered by the Windows API providing that abstraction layer, I doubt that many are.
> I still cannot buy a GNU/Linux powered desktop at the shopping mall, with 100% everything working
If you tried installing Windows from official media on one of those shopping mall computers, you would likely find that significant pieces of hardware would not work. Sometimes, it is difficult to proceed through the install process due to this. (I have a laptop where neither the trackpad nor touch screen will work until drivers are installed, so you have to navigate through the installer with a keyboard or attach a mouse). Other times, it is theoretically impossible to complete the install process unless drivers are installed during the installation process or you temporarily add supported hardware to the machine. (Windows does not support the network adapter out of the box on the aforementioned laptop, so you theoretically get stuck at the network connection screen of the OOBE.) Do the same with Linux, and the only unsupported piece of hardware out of the box is the fingerprint reader. Heck, even the printer and scanner works out of the box. I haven't managed to get the printer or scanner working consistently under Windows even with the vendor's software installed. It's also worth mentioning that none of this hardware is obscure. (Lenovo Yoga laptop, Epson printer.)
I realize that this is misinterpreting your point, that you can walk into a store and buy a Windows laptop and you cannot do the same with Linux. At least from a major retailer. Yes, that is a valid point. On the other hand, the infrastructure is already in place. The thing that is missing is consumer demand.
> lets not forget current Valve's management won't live forever
I'm not even sure how that's relevant here. You could easily argue the same thing about Microsoft. If anything, the situation for Microsoft is worse since Linux, WINE, etc. isn't controlled by a single entity. Valve is, so the convenience of Steam could vanish. On the other hand, there are open source projects that aim to create Windows game launchers that are not tied to Valve.