How long should high quality speakers last?

It all depends on how the speakers were constructed and the materials used.  Cones with foam surrounds deteriorate in a few years and need repair.  Quality speakers with long-lasting materials are a different story.  My woofers were made in 1946.  That’s not a typo.  They will turn 80 next year.  And they sound fantastic.

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.