Halo is a shell of what it once was and has snubbed the players with false promises in recent iterations (couch co-op used to be a staple of Halo but they reneged on their promise to include it with the latest).
AFAIK they haven't had any good exclusive IP for Xbox in a while. They've had a few dynamite hits (ex. Sunset Overdrive) but haven't followed up on any that I'm aware of.
Best case scenario is that Microsoft embraces these properties and revitalizes them without ruining the game in the process. Worst case is they kill it completely...
I don't think this is a point against Microsoft, they owned both Halo and Bungie during the golden era. The quality dropped off sharply once Bungie decided to work on other projects and Microsoft had to put together a new team to replace them. If they're still capable of supporting game studios however they did with Bungie, it could be exactly what Blizzard needs.
IMO, exclusives are one of the biggest issues with game systems nowadays, especially now that consoles are basically just reskinned and optimized computers.