There has probably been a bigger improvement in more affordable speakers over the decades as improved materials and techniques from higher end speakers trickles down. The Kef LS50 is a good case in point where the driver technology is much better now than it was than when the Uni-Q was introduced several decades ago. Also, as a simple fact, offshoring manufacturing has improved price/performance ratios. Note that I am making no comment on whether this is a good or bad thing, or any shade thereof.
As a final comment, when you look across various manufacturers, you sometime come across u turns and blind alleys where through a change of designer or strategy one generation of product may be worse than the one that preceded it.
A company like Wilson Audio (for example) has avoided this by having a very coherent set of design principles and then continually refining what it does based on those principles. As a result, there has been quite audible improvement in the speakers across generations.
However, the older Wilsons are still great buys used as they were fundamentally right to start with. The same applies to the various BBC based designs, and the Quad ESL 57.