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I'll run the numbers; just ballpark calculations.
A single general-purpose motherboard with a single extremely multithreaded CPU and a PSU costs a whole lot less than a 2kW miner, right? A Ryzen 5950X costs around 600€, so with motherboard and PSU under one grand. Instead of a $10k S19, you could get 10 of these, then. They have 16 cores and 32 threads, so they should be well suited. In idle, it pulls just 54W, so at the wall, with RAM and everything, it will be 60W. Ten of these will be 600 watts, instead of 1 or 2 kW.
Definitely an improvement, but not the order of magnitude type improvements that one might wish for when thinking about 'idle mining', am I right?
Edit: It's also not about physical size really; if you're concerned that 10 miners take up more space than 1, they will probably put them closer together (less heat issues with low-power miners) and / or build more facilities.
In Proof of Sleep/Time/Idle (I really want to use Sleep but I resigned to the fact that the acronym would get confused with Proof of Stake) we don't need CPUs that fast because the cores will be asleep the vast majority of the time, even when you take into account the dozens of containers running in the background - ultimately, make the sleep time long enough and they will collectively yield some processor time to idleness (there's a memory limit to max #containers).
So instead of a Ryzen or a Threadripper, old Opterons and Xeons that draw less power would be more favored. Clock speed doesn't matter at all here, just the number of threads - which you can quickly multiply by accumulating several cheap processors. There will be no motherboard or PSU shortage because there are several vendors who make generic parts for servers & server CPUs. Theoretically you could design a chassis that squishes together multiple machines in the same chassis, but that would suck up too much money in R&D - It would be far better for manufacturers to simply use horizontal 1U/2U/3U etc. racks instead.
So as long as the servers stacked vertically (or horizontally) on each other draw less than 1/3 the power of ASICS, energy is saved, assuming three of these take as much space as a single ASIC. This should be the case in practice considering the enormous power that popular modern miners such as Antminer S17 draw (over 2kW in the case of S17).