Design age isn't the defining criteria for loudspeaker quality. There are many examples of older designs that still perform at a very high level; if you happen to own one of them, age itself isn't the problem, or perhaps more precisely, isn't a reason by itself to change to a newer model. Your Vandersteens are a good example; they are excellent and are able to compete against contemporary examples from other manufacturers.
Maybe it should be pointed out that not every outstanding speaker is recognized as such. You could have bought QUAD ESLs 50 years ago and never changed, and you would still have a speaker that out-performs modern examples by some criteria, if not every metric.
I personally don't have any plans to replace my 20 year old loudspeakers anytime soon. I can do better, but not without spending considerably more money than what I paid for them. All the speakers of similar price point I've auditioned are not significantly better.
I do believe that modern loudspeakers have improved more than other component categories over the last 20 and especially over the last 40 years. However, the improvement has been overall in the market; in other words it's harder to buy a "bad speaker" (whatever that is) today because the typical HiFi speaker is better than the typical speaker of yesteryear. What isn't true is that the outstanding speaker of yesteryear has been eclipsed; they still are stellar performers.
Perhaps it's important to keep in mind that consumers don't generally buy the best speakers available in the market, and if they did, there would be much less need to "upgrade" over time. But that's not how the market overall works. A lot of ordinary speakers are sold every day, and those ordinary speakers are better than they would have been two decades ago, but they are not the best available.
Maybe I'm not the ideal person to ask the question of; I believe in providing a speaker with the best available source, and the best amplification. There are many who suggest spending the majority of a budget on speakers and leaving the source and electronics to whatever is left.
All I can say is when you read a review of any loudspeaker, they are hearing them as I suggest with high quality electronics, cartridges, and so on driving them. But it's a difficult sell, and unless someone demonstrates the difference so it can be heard in person, most buyers won't accept that advice.
Throw in buyers who don't audition anything before buying online and you end up with a lot of mis-matched systems that under-perform. But I'm not going to talk those buyers out of it, so I don't try.