How long do good speakers last?

My Acoustat 1+1's electrostatics are doing quite well, thank you for asking. I had to refoam the RH Labs sub but it also is doing well. Call it 30+ years for all of them... so far!
I have to go retrieve my dad's B&W Matrix 800's... I'm HOPING they'll be okay as they've gotten very little use in 20 years. *fingers crossed*
Happy listening.

There are 3 variables affecting aging: 1) Materials and manufacturing quality (or lack thereof) used including glues, ferrofluids, foam surrounds, etc. which can age over time. 2) Environmental conditions including temperature and more important humidity along with potential detrimental lighting effects (UV).  and 3) Damage based on usage = high SPLs that may damage driver coils and / or crossover components.

Late 80's B&W 801 matrix 2 which, aside from replacing grill fabric due to original owner's music loving cat, have required zero maintenance in over 30 years.  I keep the fabric on to prevent UV damage as the room has natural light.

I have a pair of sonus faber Guarneri homage, circa 1999, still in perfect condition. However, SF speakers require some yearly maintenance. The leather will fade if not conditioned properly & the wood needs TLC to. I would contact SF & get their maintenance suggestions. The leather will have issues if not maintained properly, fading and potential cracking. I leave mine covered when not in use to protect the wood.

I have B&W DM1600s from '86 and Matrix 802s S2 from about the same time and they are going strong with no sign of any issues. The 802s were my mains until I bought a store demo set of Wilson Sophia 2s about 12 years ago, so they are 13 yrs old and still look/sound like new. The DM1600s replaced a pair of Technics SB4500a's from '77, and a friend now has those and they are still in great shape and still rocking.  I think you can easily count on several decades with care!