How long do good speakers last?

My Altec's with cloth surrounds are original from late 60's or early 70's, I prefer treated cloth surrounds to even butyl rubber. For foam surrounds, I use mink oil about once every 2 years to keep them pliable.

I seem to have a thing for British Speakers, my original set of Tangent TM1's circa 1976 and replaced with Tangent RS4's from around 1978, I am still listening to them as I write this (Santana Caravanoral), some 40 years or so and still going strong. The drivers are all original and look in great shape, still sound great too. I have tried to keep them out of direct sunlight but they have had a rough journey.

My new adventure in British speakers are a pair of Wilson Benesch Square 2's, which I have had for less than a year, still breaking those in, but starting to come alive. Here's hoping they last as long.

I've 3 pairs of Acculab speakers, 1979 vintage.  No issues whatsoever to this day on 2 of them.  Replaced the grill cloth on 1 pair. That's it!

I bought my ADS L1590 tower speakers in 1996. This year had a complete rebuild done including crossovers. They sounded perfect before I started the refurbish. I was being cautious for upcoming years. I liked the softer sound before the rebuild. Now they sound like factor spec but so different. Going to try a different amp preamp combo for sure. Maybe quad 405s or a Schitt Freya plus 
Enjoy your new speakers. Takes a bunch of money to purchase and get speakers back to new condition!

My PSB Stratus Gold I are going on 15+ years and still are in great shape . No change in sound and no woofer surround rot , so far.