In the last 20-30 years, what has changed most...

Preamps have more moving parts so they tend to get more wear and tear . Designs for the most part are probably of a more simple and straightforward basis in preamps. Less is more kind of thinking. Gone are tone controls , phono options, contour buttons, high and low filters. Remote control has been the biggest addition in the preamp market. Less tuner / preamps are out there also. Amps seem more similar through the years. Binding post improvements are usually better.

Couple different things going on here. First, both amps and preamps have made important progress, but in different directions. Amps sound better now. Preamps do more/different things now. From purely a sound perspective, I'd vote for amps having made more important progress (especially in the power department versus 20-30 years ago).

Secondly, preamps age poorly compared to amps. The preamps of 20-30 years ago used electro-mechanical switches, buttons, knobs, etc many of which have broken or taken on undesireable electrical characteristics. These have lost their resale value.

Amps are more exclusively electrical devices, often with only one moving part, an on/off switch, which may not be used very frequently. The first things that go are typically the capacitors, which can last more than 20 years in many cases. They are also relatively easy to replace when necessary. Amps last longer than preamps. This is true for tube amps, too, so long as the tubes can be replaced.

If we are discussing brand new product, this becomes a design and parts quality issue and not just endurance to have survive the decades.

I would choose the twenty year old amp matched up with the best modern pre. I actually get excited about several of the old amp designs but do not share that emotion with old preamps.