Halo 5 also went double the framerate, that's a jump too which you aren't taking into account but should.
Had Halo 5 stayed 30FPS, you'd have undoubtedly seen a larger gap in line with previous jumps.
Sure, there are obvious improvements in visuals and performance, I don't think anyone is arguing that there aren't going to be massive improvements as hardware gets better. But the leaps are still just not as pronounced as they were.
Ray Tracing for instance is the future and I am here for it... but we are talking about real time reflective surfaces and lighting/shadowing instead of "faked" reflections/shadows we have now. It's going to absolutely be a huge improvement in overall IQ. But it's a more subtle shift than back when polygon counting of models was the rage.