The thing historically about GPUs has not been the actual lifespan of the hardware (at least half of the hardware will probably work fine for 10 or more years) the problem is that work/watt is dropping for newer hardware, so there's a point where even if you had an equivalent quantity of 10-year-old GPUs, powering them for some period costs $40k and you can buy a single brand-new GPU that costs $40k but only costs $20K to power for the same period which is less than a few years.

I don't think we're seeing any decrease in supply though, ignoring 2020 I'm pretty sure the number of GPUs manufactured has been steadily increasing. It might be the case that projected manufacturing was higher than what actually happened, which is not the same thing as a decrease in supply, but companies like Amazon will talk about it like it is, and from the standpoint of their pricing it essentially is.