I will join you in temporarily ignoring the reality that a majority of people on a major highway speed to some extent.
Even in your world where nobody could possibly exceed the limit by 1mph, then how would your hypothetical situation be a problem?
If the middle lane is doing 70 already, then its occupants are going as fast as humanly/legally possible, correct? So what does it matter about the left lane being pass only?
If the left is reserved for passing, then either those extra drivers from your hypothetical 70mph left lane have room in the middle lane to also do 70, or they don't, because the middle lane would be too crowded, and the middle lane speed would drop below 70, which means they could now legally pass in the left without violating the sacred speed limit.