@dymaxion @suzannealdrich @cstross
I was going to say the same thing: none of this hardware is usable (never mind useful) in a homelab.
With that said, 3-5 years is, I think, not unrealistic. We have racks with 8-9 year old GPUs and they're still in constant use today.
While it might not be used for training, they could still be used for other tasks, or find second life in academic HPC centers.
With Moore's Law on its last legs, they will likely be useful for a long while.