Nowadays, old server processors have very poor single threaded performance compared to recent desktop grade hardware, and the much larger amount of cores fails to compensate for that in pretty much all workstation loads. Especially considering that you can get a lot of cores in desktop hardware nowadays too. The relative value of desktop hardware vs server hardware has gone up substantially in the past 5 years or so, since Intel started making server chips focused exclusively on the demands of large scale data centers and AMD started normalizing high core counts in desktop chips.
Because cheap second hand server chips are sourced from data centers, that inherently favor high core counts and only baseline single thread performance, I don't see a future where we have cheap server processors ever being a great choice for a workstation again. Quick-ish single thread server chips do exist (e.g. Epyc 72F3), but I think those are too niche to ever see them for a good deal on eBay.
I’m certainly a power user and suspect that additional cores may actually benefit me, but there are some very interesting points about clock speed from you and others.
In 2017 buying old Xeons bought you pretty good performance for the buck, as Intel sold you quad cores with minor single core and multicore uplifts for a lot of money, while at the same time old Westmere or Sandy Bridge Xeons had a pretty good balance of many cores with decent enough clock speeds / single core performance. Even I/O options did not improve by a lot in those years.
In 2022 buying old Xeons means you get stuff from the era where Intel started heavily deprioritizing single core performance over having a lot of cores in Xeons. 14+ lowly clocked Haswell cores still do well in synthetic benchmarks, but I don't think they're worth it for any real world workstation loads. Low clocked Haswell cores are outperformed by such things as the Alder Lake efficiency cores (easily) and even modern Android phones. The latest Zen cores and the Alder Lake performance cores will be worth at least twice the per core performance of one of those individual Haswell cores. Even if you say compile code all day it's probably not worth buying into a 200W behemoth that runs Firefox slower than the Android phone in your pocket.
This blog was written in 2019 and was probably already not great advice then, but it is legit bad advice now.