It's hard to say. I agree, it seems like the MAU data for each of League and Fortnite is roughly the same as MAU for all of Steam (which is nuts). Of course there's no way to know how much overlap is there. Still, both of these titles would be a hard stop for people deciding whether to switch to Linux.

As for msft themselves though, ironically I don't know what titles they have that keep players on windows. Battle.net works on Linux, Minecraft Java ed works on Linux (not sure about bedrock ed compatibility or player count, but afaik most of those players are on non-PC platforms), all their zenimax titles are sold through steam and work great on Linux. CoD might be their biggest hold.

I disagree on number of games, but I agree on player count. The number of PC games that are not on steam (or don't work on linux) is tiny these days. But the number of PC gamers who don't need steam, or need something that doesn't run on linux is probably still quite high. Still, even if valve was able to push a few % of PC gamers to Linux, that would be huge. We're currently at 2% on Linux in steam surveys. I could see a power move by valve around win10 eol bringing that closer to 10%.