If a decent opener, with a properly increased chance to really keep his wicket intact for a while, strikes at 70, all is fine. Obviously, he should accelerate if he survives until the later part of the innings. So his rate should indeed average towards 80 then. But to state that a top three batsman should be nearing 90 is a bit of nonsense. It the top three's task to lay a platform, either for themselves or for the middle order. Such a platform can indeed be constructed by fast runs, but also by keeping wickets in hand.
Considering the last 10 years, (minimum of 1000 ODI runs), there are only 18 batsmen with a
strike rate over 90. Of those 18, only 6 average over 35.