They could easily port a Halo game on PC to gain a huge amount of money, or heck why not even buy out the guy, I don't think he would be that expensive.
It's not all that easy. The games built off the UWP platform, yes, but those not built on the platform, no. Also why buy out Eldewrito? It's not like he made the game itself. The client was already made by 343/SI for a separate project and he simply built off of a well established foundation. Licensing him the IP could be an option but that's not my call. Provided we do release MCC + 5 on PC with the builds that may or may not exist already, giving him his project would just be redundant.
microsoft is missing out on hundreds of millions not putting halo out on pc
Given it's one of their flagship IPs and they have a game console can you honestly blame them for maintaining it as an exclusive for as long as possible for the sake of encouraging console sales? Sure at some point one can argue that after a certain period of time passes maintaining exclusivity is silly, but then again all the games except Reach were moved from the 360 to the Xbox One with MCC, effectively making it a re-release.
Halo 6 also may or may not have Windows Play Anywhere like Halo Wars 2 did, but I wouldn't bet on it. Just like I really wouldn't bet on the PC community really hopping on the Halo dick train and MS recouping what was spent. They got bit in the hand with Halo 2 Vista (granted it was their own folly), but regardless the community can be finnicky. If it's not *just right* or they don't have a loose enough leash they'll have a fit. The PC gaming community is honestly a bunch of all bark and no bite type of gamers. they'll whine for something, and either not touch it with a 10 foot pole or they'll bitch that they're not given a bunch of latitude in what they could do with the game. Hell, maybe Eldewrito could get blessings for a custom edition of the games or some shit (I mean this is a real stretch but hey lets stretch out some anuses).
Also the ghost builds may or may not be cross platform at the moment but I can guarantee that if the games do end up getting an official endorsement to move onto PC it won't be. Just in case anyone was bonering up to stomp console nerds.