This is a good point. I think this won't happen though, because smaller nodes are that much more expensive, and the market for energy-efficient 32bit CPUs isn't nearly large enough to fund development. There likely won't be another ARMv7.

What I think will happen is that the huge amount of money that goes into developing mobile phone SoCs, which are clearly trending towards 64Bit while still requiring low power consumption, will cause that technology to trickle down and be used in ever lower cost markets as time passes and those initial costs have been recouped. The Apple M1, as a system, is clearly crazy power efficient, even at 5nm. And again, if you need an ultra-low power sleep mode, because you only wake up once a day to send some sensor data out, its probably not running Linux in the first place.