How long should high quality speakers last?

From the ZU's Definition IV User's Guide: "100 years; cabinet, and drivers, including the Zu 10” drivers; even in direct sunlight".

Play on!

100 years... no need to worry.   You should be able to pass them down for generations....  

Speakers last a very very long time especially if used carefully and not over or under driven.

My speakers are 25 years old, no deterioration of sound at all.

If the drive units have non foam surrounds, twenty years plus. Beyond that, some crossover components might need refurbishment.  That's assuming the speakers are driven within normal parameters - not used as PA for parties. That's for moving coil speakers.