Buy a pair of ten year old $40,000 speakers for $4,000 or new ones at that price point?

My two cents for what it's worth.

From what I've seen and read, there is a smaller return for the money on more expensive items meaning that the differences in really expensive stuff is realized by people with particular tastes and the equally expensive equipment to bring out the best in expensive items. 

To answer the OP's question, it may well be that a $4,000 new speaker gives much more bang for the buck than a really expensive speaker with the OP's taste and the electronics on hand. 

On the other hand, for those regularly bitten by the upgrade bug, buying beyond your current equipment's means could be the more economic route. 

Personally, I stay within a certain price range though that price range seems to tilt up a bit each year. Perhaps, though, the answer is to just listen to the two hypothetical speakers and pick the best for you.

As for Dynaudio Sapphires, a demo pair was the speaker in 2011 to replace my beloved ML SL3 electrostat hybrids that server 16 years as audio mains (now H.T.).

As good as the Sapphires were, for me they couldn't touch the pure sound quality of a pair of mint used Raidho D2s in 2016. The D2s were worth every penny paid (about 1/3 after 6? years). I never once considered a new speaker for same the money; none I had ever heard after years of shows could keep up (except sheer brute volume, but not on my list of concerns...).

(I'd have killed for D5s for $17k?! but would have needed a bigger home!)

This is easy to answer

If what you’re asking is are $40,000 speakers from 10 years ago better than $4,000 today speakers - the answer is likely yes.  Especially with inflation.