How long do high quality speakers "last"

44 year old pair of EPI 70’s still running strong; all original including the surrounds. Pretty sure the Pioneer receiver they run on is just as old and with original lamps and no LED crap lights either... still original caps too. Original owner. My teenage son now has this in his room.

Recently I saw a pair of Duntech Sovereign 2001's with all of the bass drivers worn due to age. The cone surrounds crumbled into dust in my hands! Now whether this was caused by sheer age, or due to excessive sun exposure I don't know...but the result is the same. Someone can probably replace the drivers...but it is going to be a project.

Not a big deal. I'll take them for cheap.......

I purchased a pair of B&W DM1400's new in 1985 and have since used them almost daily. They have traveled across the continent with me and now reside in a smallish Mexican town. Since my hearing cannot possibly be as good as it was when I was a young whippersnapper I would love to be able to compare them to newer speakers of similar quality. To me they sound great with no discernible degradation in sound and have never required any repair. When buying them I auditioned, in person, many speakers at many audio shops over several weeks. At that time , in every major city there were real audio stores where you could do this. I was looking for a "natural" sound with the smallest amount of coloration. 
Since I cannot audition speakers here now, I have no way to compare the latest technology to know if my vintage(?) B&W's still remain valid as a true natural sounding set of speakers although they are 35 years old. Can a good set of old speakers be as good as current good ones? 

My approx 40 year old Celestion SL6S speakers (a 2014 eBay purchase) are in great shape, and sound fantastic.  Given the shape of other Celestions I've seen on eBay, I think I got lucky there.  

My 25 year old Apogee Slant 6 speakers (purchased new in 1995) recently underwent ribbon tweeter replacement, primarily because I don't know how long yet Graz will be making the replacements, and I want to keep these speakers going as long as possible.

My 15 year old Realistic center channel speaker shocked me when I removed its grille after 10 years of non-use; there were no woofer surrounds, only a pile of powder!

There are so many factors that come into play that I don't think you can predict a lifespan.  Just look after them well, fix them when the break, and enjoy them while you have them, realizing that nothing is forever.