It never works that way though. Developers have incentive to make their apps work with the widest range of devices possible and, historically, most people wait to upgrade their iPad until they absolutely have to. So the majority of Apps being released today are probably still targeting A9 or A10, maybe even A8, chipsets from 5 years ago.
Also, putting the M1 in the iPad Air would actually be more stagnation than putting in the A15. They say M1 is based on A13, so there are a bunch of improvements, efficiencies and capabilities the iPad Air would miss out on that make a lot more sense to have in an iPad than raw compute horsepower that would be wasted 99% of the time.