Perhaps it already has hit it's biggest peak, but there will be another peak around Halo 5, Halo 6, they may both have peak times. Whether those peaks beat Halo 2 or Halo 3's peak is up in the air really. This is why I certainly hope Halo 5 smacks a Home run in terms of Sales and has the ability to keep it's players playing for a Loooong time.
I do know this series has hit a rough course no doubt, but all I ever hear out of people is that Halo is the only reason they will ever buy an Xbox One. I mean after nearly 13 years of Halo, and mixed rough road bumps all over the franchise (mostly in the last 4 years), it can still sell a $500 Console on it's own? This series could survive a nuke if that's the way it goes.
However... Halo 5 will be a big revealer upon how long the series truly has left. The feedback will be tremendous, and I pray it be positive. I just don't want what happened during the week of Halo 4's release to happen again. Thinking of that scenario alone will make me anxious the day before, and after it's release.