On Linux you really have all the same options as you did with Ubuntu 9, you can still get Mate which is basically where Gnome was then, you have a mostly similar userland, the kernel is way better and faster now, what would keep people on Ubuntu 9?
On the other hand, Windows seems to get more resource intensive with every release and the UI gets worse, why would anyone want to upgrade other than EOL?
A more relevant question would be if modern Linux distros support the hardware of back then (they tend to).
Or to phase it another way: Steam on Linux has worse backwards compatibility.
Windows folks don't upgrade because upgrades change a lot, newer os is unsupported on older hardware, and/or newer versions have become extremely user hostile.
Came to the same conclusion recently. And if you really want to sqeeze as much resource as possible out of your rig, game on openbox. Works wonders.
Same goes btw for battle.net (even Diablo 4 seem to work)
I'm on Linux for work related stuff since years now and switched recently entirely to Garuda / KDE
Not sure about the Proton versions TBH...
Another issue I have in linux (even in Proton) is presentation smoothness, as Windows tricks like SpecialK or fast/enhanced sync are missing on linux.
There was just an article on the front page earlier today about 75% of the top 100 games on steam running on linux, with the remaining 25% mostly being due to anti-cheat software that isn't updated to support linux yet.
Protondb.com has a database to search for compatibility and if adjustments to command lines or a specific version of Proton are needed for getting games running.
https://boilingsteam.com/75-of-the-top-100-most-played-games...
The system also does pretty well in the long tail. There's even a set of older (and some newer) games that run better than they do on modern versions of Windows - not a huge number, but not nothing and probably growing over time.
At this point, the largest problem left to solve is perhaps achieving pervasive support for anti-cheat middleware by multiplayer games, which requires vendor participation. A few have started collaborating, but there is more to do on this front.
(Disclaimer: I work on KDE and worked a small bit on the Deck at some point.)