How long should speakers last?

No easy answer to this question. Some speakers used material for the surround that deteriorated after a fairly short time and would turn to dust. This was primarily a problem with speakers made in the 1980s (I hope no one is still using that kind of material).

I have a set of horn compression drivers that were made around 1939 that work perfectly and are still very highly sought after.

My kid uses my old pair of KEFs. Still sound good to me. I am sure the caps could be replaced, but plugged into her Yamaha receiver it still makes good music.

It seems like dynamic drivers can last if the surround is made of Butyl (rubber) rather than foam. I have a pair of early-70's ESS Transtatics with KEF drivers (B139 and B110) which are still fine.

Treated cloth surround equipped bass/mid drivers are known to last decades (under proper use and conditions of course, which goes for all kinds of drivers), and when "hidden" in horns can look mint when dug out even after 30+ years. Same goes with domestically used compression drivers, many of which (also per Larryi's example above) appears to last virtually lifetimes. Less talked about perhaps is the cabinetry, the maintained aesthetics of which I gather can vary a lot. Solid wood cabinets (though too rare) look gorgeous, but the panels/lumber need a proper drying period for them not to "work" after assembly - critical particularly when used as horns. Moreover, proper veneering, where such is used, is not always a given. Cross-over components (i.e.: capacitors) seem to have improved in regards to their spec longevity since "earlier times," and where quality items are used should last decades, or so I presume.

I really feel it's what materials and glues or sealants used.Some break down some last for years and years .