Earlier quoted context omitted.
This is why Valve invested so much in Linux. They saw the writing on the wall of Microsoft becoming Apple (but shittier). Now they have an alternative. If Microsoft charges a 30% tax on all Steam transactions and won't let Steam run unless they do that, Valve can heavily push Linux and Steam Machine sales.
And yet they failed to get game devs to natively target SteamOS. As long as they depend on Proton, they haven't fully solved their problem.
kernel32+user32+gdi32+d3d[11|12]+dxgi is a pretty great API abstraction for game development. And unlike Linux desktop APIs the Win32 APIs are actually stable, so those games will also work in 5 years, and most importantly, performance is the same or better than on Windows. It's unlikely that game devs directly targeting Vulkan would do any better, and when using a high level engine, any layering overhead in Proton is negligible anyway. And don't even get me started about the state of audio APIs on Linux ;)
Also don't underestimate the amount of workarounds and tweaks that (most likely) go into Proton for games that make poor system API use. Without Proton those game-specific hacks would need to go into MESA, Wayland, X11 or various system audio libraries. At least Proton is one central place to accumulate all the game-specific warts in some dusty corner of their code base.
TL;DR: just think of Proton as an extremely low level and slim cross-platform API for games (not all that different than SDL), and suddenly it makes a lot of sense. And I bet that in 5..10 years Windows will have regressed so much that it might actually be better to run games through a Proton-like shim even on Windows (assuming Windows hasn't become 'yet another Linux distro' by then anyway) ;)