How long do good speakers last?

It depends - I had a set of speakers for 20 years and it was my moving for work several times that did them in.  Along with some help from the movers.

I think you're going to love your speakers - I have them and regularly get astonished at hearing something in a song I never noticed before.

I have my dad’s 1964 Wharfdale W70D speakers. Still in pristine condition. Original everything, and still sound good. I may replace the cloth woofer surrounds with butyl rubber. The tweeter surrounds are black felt. Wharfdale used materials that were known to last. 
Any, and I mean ANY and ALL, speaker manufacturers that use(d) foam surrounds are CHEAP companies! They knew when they used it that it would disintegrate after a short time. Don’t buy them, unless you enjoy substandard quality. Even Polk chose to use butyl rubber because they knew it would last 50+ years with no degradation, and was stable.

I had a pair of Monitor Audio Studio 6s, and then gave them to my son last year because I bought one of the new Monitor Audio floor standing models. The speakers I gave him, are now 27 years old and still perfect (rubber surrounds).

I, too, seem to have a thing for British speakers. Had a great experience w/ATC SCM12 Pro passive monitors (mind-blowingly good sound). 

~6 months ago picked up a vintage pair of big KEF 2-ways from the '80s, 103.2s. The drivers are original, and judging by the wonderful sound, have no issues (the 8" mid/woofer is made of bextrene + rubber surrounds). I know the crossovers had been carefully/selectively worked on, with any caps that didn't test perfectly replaced.

These are 35+ years old and sound amazing. 

My pair of H. H. Scott "bookshelf" speakers from 1967 still sound great for their vintage despite moving across the continent and back, among other travels. They still look good as well.