How long should high quality speakers last?

I have Kef 107.2 That I bought in 1990.

i had them totally rebuilt 2015 sound as good as they did new.

Refurb took almost a year.

100 years in direct sunlight...sounds like I'm good. I could have the cabinets refinished if I want a visual change. 

They can bury me with my tannoys....

So much depends on design and whether direct sunlight falls on exposed drivers.  As others have mentioned rubber surrounds can go bad quickly. By that I mean that surrounds can become brittle and start to tear or crumble when speakers are as young as 15 years or so.  Pleated fabric or paper surrounds can last a very long time.  I've heard woofers that are 90 years old that are still good  Horn compression drivers can last a VERY long time  I own a pair of compression drivers that are over 80 years old.  Tweeters can last a long time without any maintenance if they don't have ferro-fluid in their magnetic gaps.  That stuff can start to dry out as early as 15 years or so and maintenance requires taking the speaker apart to clean out the old stuff and replace it with new ferrofluid.

Crossover components life really depends on they type of capaictor used.  If the crossover employs modern film caps or ancient paper in oil caps, they lifetime can be extremely long.  I am having a crossover made which will be using paper in oil caps that are 60-70 years old.  Electrolytic caps might not last long, but, it depends a lot on the make of the cap.  

My Krix Euphonics were purchased in the mid to late 80's, they were demonstrators from a shop that was closing. The surrounds are butyl type material, not foam so no degradation yet.. For a period in the mid 90's I drove them with a pair of Carver M400t amps, safe to say there was abuse then... Now they are powered by a Carver M500t, modded and overhauled by Nelion in Mich. My musical tastes have matured, as has my need for speed. They are still outstanding speakers, and can probably keep going, But I'm planning a move, and have a limited budget, so maybe a used pair of Dali or Monitor Silvers??? I prefer floor standing towers... My little tube system in the bedroom runs a pair of older Totem, Arros.