How long should high quality speakers last?

Until they no longer sound good to you, or you notice that "something's wrong".  I had to replace a tweeter on one of my speakers a few years ago; both recognizing the problem and accomplishing the replacement were "no-brainers".

My Hartleys are 45 years old, sound great.  I ripped the x-overs out and use a Marchand electronic x-over (and four Pass lab amps). They have butyl surrounds as well.

They'll outlive me...big enough to bury me in one of them.

Regards,

barts

Good question!  Good replies!  Good post!  Thanks!

Infinity Renaissance 90's are about 30 years old and sound perfect.   Bought maybe the last EMIT tweeters from the mfg around 2000 and never used one.  Bought a spare set of speakers and sold them after 10 years.  In my late 60 's so it'll be a test as what lasts longer.  

More modern, polyether-based foam surrounds are very durable, and when treated with a "foam guard" should last beyond 25 years. In any event direct sunlight is best prevented from reaching exposed cones to extend longevity of the surrounds and avoid potential discolouring of the cones. When speaking of foam surrounds it seems cone movement can be a co-contributor to deterioration, likely only with older polyurethane types.

Compression drivers, as stated by @larryi, are very long lasting. Some older compression drivers with aluminium diaphragms and integrated surrounds were marred by metal fatique when crossed low (like with high output cinema use and crossed at 500Hz), a problem that was avoided with the advent of titanium diaphragms. However in a home setting compression drivers should see neither thermal nor mechanical malfunction for a virtually unlimited lifespan. 

Indeed many older, especially pro-oriented drivers have proven their worth even after several decades of use, refurbished or not. Either way it demonstrates they are very high quality designs that easily stand the test of time, in quite a few cases in a superior fashion over modern day equivalents.