IMO, sensor improvement related to better high ISO is not crucial for APS-C only.
IMhO there is an absolute
requirement for usable iso, at least for 90% of things people shoot. Yes, with super-high dr or extreme iso you can either shoot in the dark, ultra-fast movements or in any high contrast - but that doesn't happen that often.
Since the current crop sensors cannot compete for available light indoor shooting of moving subjects, for ok light I find a good iso 1600
crucial, and unfortunately this is just where the current 18mp crop sensor falls short. So my benchmark for "good enough for forever" would be +1.5ev better performance than now, but neither the d7100 or the 70d will be there yet, and if only because they trade more mp for more/same noise.