I don't think that's true for well conceptualized fictional universes (of which I believe halo is one) - but storytellers can run out of good ideas. It may also be possible for bad storytellers to close off options for storytellers in the future. Because of those universe-closing storytellers, that's when the demographic can start getting bored.
Gears of War, for example, I think is a much less capable universe than Halo (which is not a bad thing, of course) merely because the plot focus was always around the events of the games. They attempted to expand the universe to lengthen the franchise. Some love it, but I don't think the EU has nearly the visibility and reach that Halo does.
Halo on the other hand has always hinted at more. Just more. There was a rich history, and not just human history. So inasmuch as ancient history remaining ancient mystery is a Good Thing, there was always room for more storytelling. As long as the stories don't effectively cut off that Bigger Picture of the EU and always the hint at More.

Right now, I still believe that Halo has enough wiggle room to remain a strong brand, and rich universe for storytelling, with unique characters, events and places, without going stale. As long as there are Good Storytellers. who understand the conceptualized Halo Universe (not just telling a 'good story').