The majority of gamers aren't playing halo/gears/forza though...
With how saturated the market is now, they aren't reach the same numbers like they were during the 360 days with Halo and Gears, except I would say FH4 is more popular than any Forza game before, but all these franchises are still in the top played every week on Xbox and still see high engagement.
Forza is Horizon at this point, which is not a bad thing at all and good for the franchise imo. I hope Halo Infinite single player is excellent. With Halo's multiplayer being F2P I have a hard time believing it will do poorly with engagement, unless they mess it up really bad, but I don't think they will based on how they have handled MCC and Halo 5's multiplayer.
The Coalition needs to finish up the current Gears trilogy before moving on. Gears 6 is definitely happening and I'm looking forward to it, but I hope they do something new after because they are a very talented studio that need to try some new things.