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.
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.