Why have capacitors improved so much over the years?

Roger argued that the use of costly boutique parts was often unnecessary, as their claimed superior involved matters unrelated to the part’s function in a particular circuit application, and therefore provided no sonic benefit.

I totally buy this. A great deal of this is driven by armchair buyers who will nit pick relative quality and value based on parts instead of performance.

Of course, a large part of it is manufacturers attempting to make gear to sell specifically in the high end.  What if the best sounding amp on earth cost $100 to make?  No one would buy it, unless it had meters.  Big, beautiful bouncing meters.

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.

I want to submit perhaps it's equal part capacitors and equal part tweeter performance which has greatly improved.