It's doomsday if Linux starts outperforming Windows. If SteamOS for PC still required me to dual boot - which I already do - but guaranteed is get 100% windows performance or better, then that would be the official end.
It's not clear to me this couldn't happen either: I am very willing to hand over the entire PC configuration if the promise I get in return is "your games will run as fast as it is possible to run them".
When you think about it, it is kind of insane that Linux can match or outperform windows when it has an extra layer translating the system calls though. And for many of us, who don't play competitive twitchy shooters on a high level, the performance of gaming on Linux is perfectly adequate currently. I played Baldur's Gate 3 on Linux earlier this year for example, and it maxed out the frame rate of my monitor.