You can put in parts and effort to maintain older computers, but when you can buy a brand new desktop for $120 or a few year old one for $40 and they blow it out of the water on performance and power efficiency, what's the reason to do so? (I'm asking in the context of a main computer, not something to run old games natively or be nostalgic about.)

A computer you buy today will be much more viable in 20+ years than a 20+ year old computer is right now. We were still in the extreme part of speed and density growth back than, and it will lessen every decade.