> People will be able to buy those used GPUs cheap and run small local LLMs perhaps.

Maybe; I find it unlikely though, because unlike CPUs, there's a large difference in compute/watt in subsequent generations of GPUs.[1]

I would imagine that, from an economics PoV, the payback for using a newer generation GPU over a previous generation GPU in terms of energy usage is going to be on the order of months, not years, so anyone needing compute for more than a month or two would save money by buying a new one at knockdown prices (because the market collapsed) than by getting old ones for free (because the market collapsed).

[1] Or maybe I am wrong about this - maybe each new generation is only slightly better than the previous one