Linux

Linux and Windows are, I think, too different conceptually

That is correct. Linux is a UNIX variant/clone, one of many. UNIX isn't just an OS, it's an entire concept on how to build and organize an OS. People keep wanting it to be Windows, but it will never be Windows. I wish it could just stand on its own as a flexible, open, UNIX-like system with its own strengths, not just be a drop-in replacement for Windows on the desktop. Let's make the UNIX way of doing things the normal, everyday way of doing things.

Some applications require relatively obscure userspace quirks and tricks, in which case it doesn’t matter if they’re running through Linux+Wine or ReactOS+Wine if Wine doesn’t implement them.

I thought the whole goal of ReactOS was to get to a point where wine isn't needed at all. You just take the .exe installer, and install it. Just like windows.

The issue is that they're so poorly staffed that they're waaaaaay behind on what Windows considers modern. They're doing an impressive job reverse engeneering, but it's still a 20 year old system. They partially rely on wine currently because of how unfinished THEIR work is.

But my vision is that thousands of people each contributing small little bits here and there, all contributing to the code of ReactOS. Eventually they could move beyond XP, into Windows & era, and Windows 8 era, and Windows 10 era, and Windows 11 era. But since it's NOT microsoft, and since it's NOT windows, it wouldn't come with any of the stuff people hate about those versions. It would come with the compatibility, but not the forced spying, or the bloat, or the forced subscriptions.

The thing I read is that Windows 95 was composed of around 1000 people all working towards the same goal, with the same tools, at the same time. All getting paid as their full time job. ReactOS has about 30 people, who are all fighting with each other. Their last stable release was 2021, but they do have nightly builds that are "current". But with so much infighting, and so few people, and not being a full time priority, it doesn't get the support it should.

As far as what linux would gain by ReactOS reverse engeneering modern day OS's? It would mean ANY program that was originally intended for windows could be installed via the .exe file, on ANY linux distro. Using the same gui setup that windows users know how to do. Which in turn would make it 1000% easier to convince a new user to switch from Windows to Linux, if you could say "You can run all the same programs, you already know how to use it, and it's free." THAT would be the game changer.

what applications would run in ReactOS that wouldn’t run through Proton/Wine?

Drivers and usb hardware support are the main things ReactOS currently are better positioned to support that I know of.