How long should high quality speakers last?

If a speaker is about 10 years old, you should test the drivers, crossover, etc and determine if everything is in spec (even if you weren't abusing it).

If you plan to keep some speakers for a long time,  get/keep some spare drivers on you before some manufacturer leaves you stranded...

I would not even think about this issue for about twenty years. I would examine the woofers edges for deterioration of the tape between mount and cones. Nothing wrong with examining it early. I bought four 800 Series B&W speakers and ended up getting them all re-taped at around 25 years. Other than that, they should be good until you want better performing speakers. Typically for me that is every 10 - 15 years.

It really depends on the speakers.  Foam surrounds can rot after 20 years or so.

Mine have butyl surrounds and are 35 years old. 5 years ago I refreshed the ferro fluid in the tweeters. They’re as good as new...maybe better because the caps and woofers have thousands of hours on them.  Hoping to get another 30 years from them. 

It’s a good practice to rotate your woofers 180 degrees every decade or so to offset for any gravity pull.

I think on their site Zu says "100 years"     Enjoy

My speakers are forty one years old, they look and sound like new.

Mike