Why have capacitors improved so much over the years?

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.

While film caps have focused a great deal on reducing ESR (Equivalent Series Resistance) we've had little to demonstrate that alone makes for a great cap.

On the electrolytic side though I think there's a number of improvements that are hard to dispute.  Better longevity and reliability along with lower inductance and lower ESR.  I won't say the need to bypass them is entirely gone but man compared to their brethren from the 1980s things here are really so much better.