It's way better now than it was back then. There was a long period of good ports, which combined with the Steam for Linux client made Linux gaming a real thing already. But instead of fizzling out like the last time there were ports, now Linux transitioned to "Run every game" without needing a port. Some exceptions, but they are working on it and compatibility is huge.
This will grow slowly but steadily now, and is ready to explode if Microsoft does on bad move (like crazy Windows 11 hardware requirements, but we'll see).
Biggest danger to that development are the gpu prices, the Intel gpus can only help there. A competent 200 bucks model is desperately needed to keep the PC as a gaming platform alive. It has to run on fumes - on old hardware - now.
Vista brought a lot of migration to Linux indeed. And the Mac being incapable of playing games is a factor for quite some people. But until recently, gaming on Linux was limited.
What's the rate of increase though? Has it been linear, 0.05% per year? No: it has increased about 0.25% since 2019. It's not just that Linux is increasing, it's that it's rate of increase is also increasing.
How is it wishful thinking? It's a proportional increase. Meaning that it has increased in both absolute and relative terms, and done so at a faster rate in the last two years. I'm stating an observation of the data. I don't see room for wishful thinking in that.
Other platforms can increase while Linux also increases, the two aren't mutually exclusive.
I'm not saying it's going to take over the market. It's a huge long shot it will ever really rival Windows. Even if makes significant headway on Steam, that also doesn't necessarily translate to a corresponding change in OS market share. But the pending arrival of the Steam Deck combined with an enormous increase in game compatibility has nonetheless set the stage for significant gains for Linux with Steam.
Which part of the above observations are wishful thinking?
I believed in it back in the Loki golden days, nowadays I rather bet on macOS, Windows, mobile OSes and game consoles.
It remains to be seen how the Steam fairs versus the Steam Machines of yore.