Given that there are a lot of cases that old systems are used for that don't run the system full out and that they probably idle a lot when on, just like your new stuff, given that a lot of older hardware barring known stupid designs like Pentium 4 that draw ludicrous power for the perf, and given how much of an absolute shit show electronics recycling is (I've seen a good amount of how it actually gets "broken down"), I honestly think we upgrade way too often for most use cases. If you can still get by doing something important with a P3 or a Core 2, honestly I would be really surprised if it was actually vastly more cost efficient and environmentally friendly to refresh to new hardware.