The biggest threat to Windows is Steam

[G2ThaUNiT]

I can see that eventually being the case. Valve tried that a decade ago with Steam Machines, but there's still major issues that Valve would have to overcome as well since it's not just Steam on PC. Very few games are made natively for Linux (underlying OS for SteamOS) so Steam Deck operating has been reliant on a Windows compatibility layer called Proton to get Windows games to work on Linux. Unfortunately, even to this day, tons of games on Steam do not work with Proton.

So Proton is a short-term solution. It's not the long-term solution. Valve would somehow have to get all developers to make native Linux versions of their games for mass adoption of the OS. Because outside of Steam quite a few publishers have their own PC storefronts that either do not work on Linux or have a lot of/all games that don't work on Linux such as Ubisoft, Epic Games, Blizzard, EA, Rockstar, and Riot Games.

Then there's the other issue of Anti-Cheat as multiplayer games take up a massive amount of PC gamers, nearly all of them do not work at all through Proton. So we're looking at many years before this type of scenario comes to pass.