How long do good speakers last?

depends on the speaker and their construction, most last quite a long time if not damaged

as the poster above mentioned, worse come to worse, mids and woofers may need surrounds to be redone

 Congrats on the 5’s I have 3’s,1’s and center on the way.

my current Revel F32’s are 18 yrs old and no sign of physical wear.
i am sure climate plays a big part

I have owned my Apogee Studio Grands full range ribbon system which were made in 1996 and they sound better that the day they were born.  They have never been touched repair wise for any reason. 

So long as they're not damaged electrically, and you keep the cones out of UV light, new, modern drivers will last longer than a lot of us.

As others have mentioned, woofers with foam surrounds may eventually need replacement of the surround.  The life of such surround is usually shortened by exposure to sunlight and air pollution (particularly ozone).  Other kinds of drivers should last longer.  These days, tweeters are not built with magnetic fluid in the voicecoil gap; that fluid eventually dries up.  
I’ve heard many systems that use terrific sounding woofers that have never had to have the surround replaced because they do not employ rubber foam (e.g. pleated paper surround); some of these drivers are more than 60 years old.  The midrange compression drivers in my system are around 73 years old.