Can anyone tell me where the progress in audio went?

Engineering at any price point is always a tradeoff. No, you "can’t have it all" most of the time - but you can have much of it. I think as others have noted that "technology" and manufacturing has reached a level people only dreamed of say in the 1950s and 1960s. As one guy mentioned this is a GREAT time to be into audio no matter what your budget is, from $1K to $1M. And you reach the point of diminishing returns pretty quickly as you go up the food chain. At some point you are paying more for the industrial design and "looks" than for a huge improvement in sound quality.

Look at digital cameras. Most DSLRs (and lenses) have reached a plateau in performance. Indeed, with mirrorless cameras pushing DSLRs into "old school" obsolescence. What more could be added? They already have more features than 99% of people will ever use, even a professional photographer. For the masses, their cell phones might be the only camera they will ever own.

And for some, it will always be (as it should) a matter of taste. Do you want the liveliness and dynamics of a horn speaker or something more refined and studio sounding. That was the answer Andrew Robinson gave on his YouTube channel when comparing the new Mission Audio 770 to the Klipsch Forte IV.

@theaudioamp you wrote, "I would argue that lenses still have a long way to go, especially when price is taken into account :-) "....My nephew who has a few Sigma Art lenses would say his are "good enough" and especially his wife, who looks askance at his spending on hobbies like audio and photography... ha!