How long should speakers last?

I've had a set of NHT 2.5i speakers for 18 years. They still play great. How long should speakers last? Is there a rule of thumb?

I am about to purchase a similar age set of mirage m3si. Assuming they were well taken care of, how long would you expect they last?

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 .
Old thread, but my ADS speakers from the mid 1980's are still going strong! Butyl rubber surrounds are the best, it appears!