If I have "audiophile-grade" speakers from 20-30 years ago, what are the differences in sound quality as compared to modern high-end speakers? (To nail this down, let us suppose we're talking about a reference speaker that sold for $1,000 a generation ago, and we're comparing it against new speakers in the same price range of the same size, power, and dynamic frequency range.)
I can think of several possible reasons why older speakers might not keep up, but I don't know if any of these are true in reality:
The state of the art in speaker design and manufacture has improved, so that the same level of performance can be obtained more inexpensively today. (Equivalently: what used to be cutting-edge designs are now mainstream and produced in larger scales.)
Speaker components degrade over time.