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

You wouldn't buy a used car that was once 40k for 4k would you? Seams kind of suspect unless of course your into total restoration then those speakers or that car might seem worth it albeit you'll be dropping a pretty penny to revive either..just go with new, if you can?..

Interesting thread. I have designed and built a lot of high end speakers, and I have owned many, and used more as a studio Engineer for 25 years. Just to respond to some of the points here. 

It's easy to get a 10 to one sort of advantage if you are willing to undertake a repair. 

Drivers last a long time unless they are driven hard all day.  Because the Voice Coil is suspended in a magnetic field, friction at the moving part is minimal. Surrounds deteriorate, and are generally replaceable. Paper cones do too, but they are also replaceable. Entire drivers are often replaceable, and not very expensive in the overall picture. The top end driver makers (Scan Speak for example) often will have a close-to identical model for some very old drivers. Sticking with a vendor who has been around for a while improves dramatically the chances of finding a proper replacement. 

The way to tell a burned voice coil is to smell it. No kidding. 
In a crossover, the main failure point will be caps. If you have older speakers you can get instant improvement by replacing the caps with some of the excellent audio caps now available. Caps in general are a horrible thing to do to an audio signal, so I avoid any crossover over 3rd order. Low order crossovers do sound better, but make other aspects of design very tricky. 

The "overheads" on speakers are design and engineering time. They are totally worth it. The basic MO of large manufacturers is to use extremely smart design and scale to help drive down the cost of parts and manufacturing. You pay the design costs once. Crossover design is mostly automated at this point, but in years past, getting all the factors in place (box, drivers, xover) just to get a good test was very time consuming. So, one advantage of more modern speakers (>2000)  is that design tools have become commonplace. The interaction of xover, drivers, and box is still a matter of engineering art, however. 

Speaker builders who sell just a few pairs of a high end model can't make much of a living. However, I expect that 3d printing, the availability of cheaper NC machines, etc, has changed that. Keep in mind that the cabinet is costly. I used to have fun buying blown up speakers with good cabinets, and reverse engineering new drivers and crossovers to the cabinet. That's a way to get really amazing quality inexpensively, although it probably makes a few people choke on their coffee. 

Finally, in general, second hand speakers are a bargain, and a chance to afford speakers that were built without compromise. Audiophiles, including myself, like to experiment with sound, and with speakers, that means selling your old ones. My old ones are worth zero to me after I get new ones, so I sell them cheap. Also, our circumstances change. Listening environment, noise restrictions, room size, SAF.  Any of these factors demand different speakers ... so, again, bye bye old speakers. They might be perfectly good, but are only worth what the market will pay, which, sadly, is not much. 

I guess I would look at the problem your seem to be trying to solve in a different way.

You have $4000 to spend, OK.  What is the size of the room, what electronics to you have to power them, what are your musical tastes all must be considered first.

Then you can look at what you can get new for 4k or used for 4k that meet those needs and make a decision.  Certainly there is always some risk buying previously owned but the site has a thriving clientele buying and selling "used" gear.

Also new speakers are easier to audition but even if looking at particular brands used, you may be able to listen to the general sound/voicing from that brand to help.

If I were you I'd take that approach, put in the time and effort and not expect a silver bullet answer 

 My 25+ year old Legacy Focus speakers are still being made, with higher quality parts, for $13,000 while used older pairs go for $2-3,000.   However, the older units sound better.   Same with the Legacy Signature IIIs, selling for $1-2,000 (foam surrounds on woofers need to be redone rubberized).  New Signatures are about $7,000.   So, not 10X cheaper but very significant bargains in the used category for great speakers.

Big heavy speakers are a challenge to sell at any age. So when someone has to move them, bargains are possible. So key is listen to them and assess their value.  But speakers do need foaming when they age, and that could be a problem. Also are parts available, is the company in business still, etc? This would greatly diminish the value. Custom updates are possible, but at the buyers discretion and caution. I love bargains. I say go for it. My Bozak speakers I gave away to a friend are selling for real money 40 years later!