How long do good speakers last?

I had ac pair of JBL Laners go 20yrs before the woofer foam started to go bad

I've got a  pair of 1959 JBL Hartsfield the 15inch woofer is still in good shape after all these years.
In year 2000 I bought a pair of Klipschorn the 15 inch and horn driver are still in good shape.
In April this year I purchase my 3rd collection of JBL Everest DD66000 used by the last owner  who had them since 2009.
I prefer  drivers are made of cloth treated surround it will last for Donkey's years.
I had many other pairs of speaker with Form surround they last 3 to 5 years in Tropical Singapore.
So I only purchase the once with cloth treated surround. I am very happy of my purchase of my three pairs of speakers in my collection and I do know they will out last me. at age 67 years.

Depends on the speaker mostly, and how much the cone moves.
If you your speakers have "long throw" woofers with excessive cone movement, re-foaming them will be a regular activity for you.

If on the other hand you own a pair of K-horns like myself, there won't be any need to re-foam because the cone never moves more than a 1/16th of an inch, worst case scenario. No movement, no wear! 

I bought my speakers new in 1978, the access panel for the woofers have never been removed. Bass is as good as it was new...

Depends entirely on if and when you go looking / listening up the food chain! There are several models up from the Olympica Nova Vs and of course other speakers worthy of a listen.
If you really want to see how long those speakers last I recommend you don’t go listen / look at other speakers. The SF’s are great speakers. However, people change!  Good luck and enjoy!

If this has been mentioned before then ignore. Although we are all supposed to believe that things made now are better because of the new materials and technology available that is not always the case imho. I use on a daily basis speakers made in 1978 and 1990 and they both sets are working fine and neither have put a foot wrong during all that time. Of course grill foam disintegrates and has to be replaced but that's all in my experience. Treating the speakers well and not subjecting them to excessive heat, humidity or physical abuse will add to their life.