How long do good speakers last?

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!

I have a pair of AR-LST speakers that I bought new in 1975 or 1976. I've had the woofers refoamed twice. The cabinets were recently refinished, and we bought new grills. They look great and still sound wonderful. The refinishing work cost more than the speakers! Should last quite awhile longer.

I have my great grandmother's mono tube integrated 78 rpm turntable and world radio that has a speaker that looks barely used and ready for anything I can throw at it.  Yet, my 15 yo center channel speaker has a completely rotten surround possibly from being snuggled up to my Pioneer Kuro plasma TV for most of it's life.  The rest of the speaker's surrounds in that system still look perfect despite having lived in the same room playing the same movies.  Replacing surrounds as an easy job, but I wonder if you'll ever have to do that with those speakers.  I think you made a great choice for a life partner!