It's in the same ballpark of power efficiency as AMD's x86 chips. It's slightly more efficient because of 7nm vs 5nm but if you scale it up to desktop frequencies it's going to consume the same amount of power as Ryzen CPUs.
So again, what would make anyone think that an M1 with decent cooling would not be able to maintain the current ST performance indefintely, or a hypothetical 8+8 or even 16+16 core M1X or M2 with a TDP of 100W and top-notch cooling solution would be impossible?
Don't forget that scaling up is also not just about frequencies, there are also packaging considerations - the CPU dies have to actually be able to dissipate the heat generated, and the package itself has to be able to do so as well. I'd expect that this is something AMD and especially Intel have a leg up over Apple with - although, considering they've already tread that ground it makes Apple's job a bit easier too.