> It still makes sense to use a decade-old computer.
I don't really think it does, at least not some of them.
A lot of those older machines are just running really, really high idle power draw. Ex: Pentium 4/5, Core 2 Duo, Athlon64, etc. Running from 40 to 100 watts idle.
For comparison, a new RPI will go as low as ~1-3 watt idle, tops out at ~5-7 watts under full load, and will generally have better performance characteristics.
I run a lot of old machines in my basement because it can make sense (I throw them in a k8s farm), but I also generally have them spend a week on an electricity meter first (honestly, just a function of the UPS I run down there) and anything drawing more than 20 watts idle gets sent to the recycling instead.
The bad ones tend to run high + hot + loud, and they absolutely aren't worth it.