The PC gamer userbase is very big, not to mention they will enjoy a better and smoother experience than on an X1. Steam is really huge. Just look at how high the FFXIII preorders are on Steam despite it being 4 years old.
Yeah but, what is the comparison between Halo PC players and Halo Xbox players during their respective launch periods? I'd wager that Halo 2 on Xbox has FAR more players online than the PC port, even when it launched. That's what I'm saying here. Even if they release this on PC, by the time it does come out, it won't draw in the same amount of players and the community will be tiny by comparison.