Microsoft's control of the Windows kernel is a strategic issue; with it, MS is in command of its own destiny and platform. Sure, MS could fork the kernel, but then you're maintaining a kernel but this time, it was written by a lot of people for whom MS's best interests really didn't align with theirs.
For a similar reason, I doubt Apple will ever transition away from Darwin, which is an open-source project that no-one else gives a shit about, and which definitely goes wherever Apple wants it to go. Indeed, Apple is so desirous for total control over its platform that it's developing its own CPUs!
("The thing about games is that they are the most demanding possible stress test for a Windows emulation layer, much more so than business software." Bullshit. Games aren't demanding on Windows itself. They're almost totally abstracted through the various frameworks - Vulkan, Havok, etc. The bit of the game that talks to the host OS is mostly concerned with input (collecting key/button presses) and networking. Games don't give a shit about your screen reader, or your system-wide locale, or whether you have sticky-keys enabled because you're motion-impaired, or whether you have fifty printers installed, or whatever)
Being real for a moment, and accepting that MS aren't going to suddenly support the entire Windows GUI layer (and all its dependencies) on Linux (via Proton or otherwise) - what about Linux itself?
LOL no, basically. Linux is just too much of a UX shitshow across the board to gain traction with normal people.
Even amongst the nerds - Steam users, for example - it hovers between 0.5% and 1% of users. Sure, a bunch of developers use it, but even then, in my experience, they're vastly outnumbered by Macverts. Even those whose development target is Linux use Macs.
ESR fails to recognise that the vast majority of users, both domestic and commercial, are perfectly content with what they've got now - be that macOS or Windows.
It's been 30 years. The window of opportunity for Linux on the Desktop has long since closed. Both macOS and Windows are very, very good operating systems with vast, entrenched software ecosystems.
As a user environment, Linux needs to be dramatically better than Windows to give people a reason to undergo the pain of transition, and it just isn't - and, at this point, I doubt it ever will be.
tl;dr: ESR is just having an idle, eyes-half-closed tug over the end of Microsoft. Worse, he's fantasising about a world where there is One True OS, which is no better than the world he's trying to replace.
After all, if we had to converge on One True OS, anyone with a brain would base it on VMS.