Halo is a $40 collection, or $10 for Halo Reach only. Its multiplayer is popular so people will be playing it for a long time.
At the rate at which its going? Its hard since usually peaking concurrent players wave lasts like only 2-3 hours but even if it can't do 200K tonight, it might as well do that at the weekend.
While reach does mess with the timeline, it really only hurt extended universe stuff... but it does tie directly into Halo CE