They'll probably add it to pc a little after infinites release. Not excited about it though, all the cool clips that people get on xbox will be easily outdone on pc, making many players lose the will to play it on console entirely. Just like what happened to mcc.I honestly think if they add it to PC after Infinite releases, no one will play it. Maybe those hardcore Halo 5 players, or anyone who wants to play Halo 5 to understand the story or unlock the Infinite rewards. But other than that, why would anyone play it? Infinite will most likely be THE Halo game everyone plays for a while. I mean, I wouldn't be mad if they added Halo 5 to PC after Infinite releases, but I think it would suck if they did.
I think people will definitely play it. Think of how many Halo games there are and people support them. People play 6 different freakin multiplayer Halos in MCC, people play mods like extinction and SPV3, people play (or will play based on the interest shown) fan made Halo games like Winter contingency and Installation 01, people played Halo Online that was only in Russia, people even play Halo on Xbox 360. The community LOVES anything Halo, specially when given the opportunity to play Halo in new and fresh ways. Think about how little people cared about Halo 4, and it's hella successful on PC. If we had the same mentality towards it we never would have gotten it. A lot of Halo 5's lack of popularity is also because of it being restricted to Xbox, it would have no trouble finding its own sub-community.
I really hate how much people dismiss Halo 5 because they judge it on it on the franchise legacy, not as an independent game. Even MS themselves dismiss it. Halo 5 is the latest in the series, the campaign, no matter how much it's criticized, is a new experience and at minimum the gameplay can be appreciated. The multiplayer is entirely fresh compared to past Halos and is considered one of the best multiplayer games of last gen by many. It has the most robust Forge mode and most people don't even know how incredible it is. It has warzone mode which actually gives you things to work towards, for those people that like to unlock weapons with their progress, and it is totally independent from the traditional MP modes.
I honestly feel that the community needs to just step up and demand it, even if they don't think they will play it having the option to play it only benefits the franchise and the community. If we keep coming up with reasons to prevent it from happening it will certainly never happen. No one expected classic Halos to be on PC, no one expected Halo 4 on PC, no one expected crossplay and 120 FPS. If they make it, people will come.