I mostly agree, and did the same with my second gen lab build - instead of shiny new NUCs like I had used round 1, I bought old off lease Dell Xeon boxes. SO MANY PROS -
- Got them up to 14c/28t each
- They can take GPUs and actually do heavy transcoding/ML work
- They can take up to like, 128GB of memory, which is GREAT when they're all hypervisors
The downsides can't be denied though -
- Even without the GPUs and beefed up CPUs, they are power hogs - the CPU alone uses more than an ENTIRE NUC
- They run HOT
- They run LOUD
The same holds true for off-lease SFF stuff, Lenovo and the likes ...
So while reuse/repurpose is absolutely of the utmost importance, no question - when it comes to technology and how quickly it advances and miniaturizes, a thorough and logical pros/cons list is often required.
I'd add another option though - if you do need what a Pi brings to the table - do you really need a shiny new Pi 5? Is it possible a used Pi 3 or Pi 4 would do the trick, and check the reuse box?