I lol-ed at first but maybe the Masterchief collection will have infinite legs. Regardless of how Halo 5 turns outm, the original 4 games with their respective multiplayers intact all remastered onto one disc is bound to be a continued seller for years right?
10m lifetime is achievable IMO. Definitely not this year or next though.
I'm predicting 7m after holiday 2015 and then 8m-11m LT depending on legs and how well X1 sells longterm.