as someone who runs a mix of late 90's, early 2000's, and a newer processor - I think your system should be a combination of what you want and what you can afford. I think speakers and processors should be recent due to advances in technology and component reliability - but amplifiers are near timeless.
i would like to run new Genesis Series III or Audison Thesis amplifiers - but i can't justify spending that much. so instead of paying $300 for a new POS amp, you can spend $300 on an "old school" amp that outperforms with lower THD and better build quality. I'm not a fan of most of the cheap outsourced stuff under $500. reliability is low, build quality is low, and sound quality is low.
people hang onto what they used when they were younger, common with musical tastes and also the gear they run. but mostly, i think it's an issue of budget. if we could all afford to spend thousands on each component, we wouldn't care about most of the "old school" equipment. sadly, we don't build $10k systems so we make ourselves feel better by holding the now affordable "old school" gear in high regard.
but each amp does sound different - so there is some personal preference in there. I personally like the sound of pre-Mitek xtant amplifiers, so i use them. it's also nice that they can be had for a fraction of the price while insurance companies give you MSRP value. i also like the sound of Audison LRx and will replace my xtant amps with them someday - but the frugal side of me will probably buy them used.