The last halo on Xbox one hardware , so for the foreseeable future , it is infinite. The next Halo will not run on Xbox one hardware and will require Scarlet or whatever it will be called. Also seeing as this is something of a soft reboot , I believe it will be the end of Chief and Cortanas story Arc and possibly the end for both characters. Which means it will be forever remembered by Halo fans ; whether it is a good game or not ; for simply being the end of an era.
The ring theory above by JenguBlocku is quite good also.
Unfortunately , Steve (Downs , The voice of Chief) is getting up their in age and with Halo trying to branch out onto Television or movie screens they kind of need a younger actor to portray the main Protagonist. Yes i know they could get whoever to play the guy in the suit and have Steve voice him , but that effects the actors around the body double as the lines aren't delivered the same (tone , cadence , delivery) , it creates a dichotomy that can comes across in the final product. It would make far more sense if they were to follow a younger Spartan on the show/movie , and move him/her over as the games main protagonist after Infinite.