How long do good speakers last?

Humidity plays a big role in how well speakers age.  We have a dehumidifier for the humid 7 months of the year and that keeps relative humidity to around 45%.  Moist enough that the seams don't dry and crack, but dry enough that mold doesn't grow and moisture doesn't destroy the cross-overs.

The oldest speakers I have are a pair of Polk Model 10 A's. 1979. Excellent speakers for their day. They still sound like they did in 1979. 
I'm skeptical of new technology that promises "better" sound. No one really knows how they'll hold up (or if the new company will survive to service your drivers down the road). There have been countless innovations that have turned into boat anchors. I stick with what is tried and true. 

I've had my Dunlavy SC V, IV, III, I and Cantatas for over 20 years. Had to replace 3 or 4 drivers along the way. Outlasted all my electronic upgrades through the years. Have never once had the urge to replace the speakers.

Oh yes, and I should include the subwoofer column in an SC IV enclosure in the two channel system.

Recently bought a pair of used PBN Montana XPS speakers. From the serial #s, they look to be from 2006. They are near flawless in every way and I don't see why they wouldn't last for a few more decades. Of course all speakers are built to different quality standards.

My Apogee Acoustics Duetta II Signature Series speakers were bought new by my dad in the early 90’s. They still sound great, but the inferior acoustic foam from those days is situated internally to where once it breaks down to where I now have a bass buzz at a very specific frequency on the left speaker (which
 UPS freight also dropped hard and damaged a bit). Plus the remaining acoustic foam can’t be replaced without replacing the ribbons. Due to this, and also wanting to take advantage of the newest Graz designed ribbons, much better crossover caps, binding post, etc. I’m having the speakers totally redone in about a month (waiting in backlog with Music Technology in VA for almost a year, pandemic really stretched this period out). Longer story shortened, many have these speakers from this era still going strong. Just depends on your what you are willing to tolerate and can afford in the long run. That said, conventional speakers are much less prone to these challenges.
The key I think is speakers can last a lifetime when taken care of and not moved (by others) as much as possible! Shipping companies are potentially your worst longevity enemy by far!