Why have capacitors improved so much over the years?

Not just capacitors. Chips, transistors, resistors... almost all electronic components had been improved over the years.
Btw, in terms of capacitors, electrolytic capacitors been improved greatly.

I have not been involved in the engineering of any audio product but If I were, I would insure that that no corners were cut on the electronics. Unfortunately I think capacitors are generally the weak-link especially in Class A amps and any given product is only as good as its weakest link. If there were a way to eliminate capacitors, that would be miraculous.

BTW...salt is not "just salt". Salt absorbs minerals from the water in which it forms. Those varying minerals give salts different flavors. Not to mention the differences in the salinity levels per volume of salt. Do a salt tasting; you'll be surprised at the differences you can taste.

Yep, all that heat in Class A is murder on those electrolytics. But who wants to rule out Class A, even for power amps?

It’s a good thing to keep the hot spots (rectifiers, output transistors) away from the electrolytics and noisy power transformers away from the listening chair. I solve the problem by putting a spacious power supply in another room, with reactances well away from the heat, but not everyone is lucky enough to have the space.

If there were a way to eliminate capacitors, that would be miraculous.

There is. They are called oscillators.

So much blather. So little knowledge.