Actually, if I'm correct, Marty stuck with Bungie after they left Microsoft and Halo to make the music for Destiny and then Bungie fired him, not 343, Marty was never a part of 343
You're correct.
Marty O'Donnell has always been employed at Bungie, hence why the music for Halo 4 was composed by different people.
it would suck even more now since windows 10 is apparently going to be a free upgrade - this f2p halo won't even be able to sell windows 10 like halo 2 PC did with vista.
Good try, but incorrect. Just because Microsoft has a large market share doesn't mean they wouldn't want to expand it with their flagship product. Windows 10 has DirectX 12 and Cortana, and if they announced Halo 3 or a similar thing to MCC for the PC, I can guarantee they would sell many copies of Windows 10 to consumers who aren't using an upgradeable OS yet, provided they market the game correctly.
There are plenty of Mac users with Xbox consoles.
so the solution to that is to do what they did with mcc and only release the old game on pc and then give all the new ones to console users
What the forget are you talking about?