How long do good speakers last?

I have B&W DM1600s from '86 and Matrix 802s S2 from about the same time and they are going strong with no sign of any issues. The 802s were my mains until I bought a store demo set of Wilson Sophia 2s about 12 years ago, so they are 13 yrs old and still look/sound like new. The DM1600s replaced a pair of Technics SB4500a's from '77, and a friend now has those and they are still in great shape and still rocking.  I think you can easily count on several decades with care!

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.