I'm impressed they're still making these games. I thought 3 was just the end, and I don't mean story wise. I mean like it was just getting overdone at that point already and it seemed like they were low on ideas. I remember beating the campaign and thinking, "What the fuck?"
I only played the original due to lack of PC versions, so I can't say, but Microsoft doesn't exactly have a strong stable of exclusive franchises outside of Halo. I doubt they want to give up any of the ones that have been a success.