If Halo 4 was similar and better than Halo 2/3 at 60 FPS on Dedicated Servers with as many or more features as Reach, then I doubt Halo would be where it is today. That would be a rock solid package that could compete with CoD.
Unfortunately, 60 FPS and Dedicated Servers weren't a priority for Halo last gen, and the gameplay that 343 crafted was terrible. But, if Halo 4 was what it should have been, with the huge install base of the 360, and on the MLG circuit, I think it would still be one of the highest populated Xbox 360 games today.
Might as well hop in on the speculating and made up scenarios.