How long do good speakers last?

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!

AllIison CD8s, 1988. Refoamed this year. Will never give them up.
Mirage M290s, 1995, still perfect.
Audio Physic from 2001, perfect. Still think of them as new.
Tannoy near fields from early 90s. Never needed work. Perfect.

If you are 51, the only issue you will ever have is if you want to get something else. Those will last until you are playing dominoes in the old folks home. Where I'll be soon with my Allisons.

My DQ10's still sound wonderful.  All original except a re-foam.  They could be 40 yo......