Tough question with many variables. If concerning only maximum attainable SQ, I don't think speakers have advanced much at all in the past couple decades. Consider that most exotic driver types have been around for 20+ years now. Still, some of the most well-regarded speakers in the new market use fabric tweeters and paper-cone woofers. I've heard speaker models 30+ years old that retailed for $3K that can give some current $15K speakers a real run for the money.
Of course, there have been advancements in design and measurement tools, but I think that's largely benefited ease of speaker design rather than speaker performance. It's not unlike the world of musical instruments, in which some of the best sounding violins and guitars were being produced well before the advent of computers. To my ears, the greatest improvements have been taking place in the budget speaker realm, where advancements in manufacturing tech and efficiencies has allowed production of respectable speakers at very economical prices.
Also consider the factory-direct pricing model that's gaining traction. I've recently realized that at least one of these manufacturers is essentially offering 100% greater value by cutting out the middle man. Their $3K model easily competes with most $6K speakers sold through a traditional dealer network.
If one has $8K to drop on a pair of speakers, rather than take a gamble on a used pair that retailed for $20K, that person might be better off getting similar value by going the factory-direct route. Most of these manufacturers are offering low-risk (one-way shipping cost) 30/45/60 day in-home trials and very competitive warranties.
Lastly, consider that just because a speaker was at one time expensive, doesnt mean it was ever any good. There are plenty of overpriced/underperforming speakers on the market.
