How long do good speakers last?

Years ago, replaced foam in Advents prob. 15 years old. Set of KLHs (very similar, no?) still going after 60 years in brutal conditions (summer home, deep frozen in winter). I did change the capacitors last year, just to do something, but I doubt they needed it. Current Focals sound (to me) exactly as they sounded when I got them a quarter-century ago.

(But, as implied above, even if the physical speaker lasts, the quality of sound relative to what other speakers are available will likely change; those Advents, which I loved, didn't sound so great once I found what might replace them).

Humidity plays a big role in how well speakers age.  We have a dehumidifier for the humid 7 months of the year and that keeps relative humidity to around 45%.  Moist enough that the seams don't dry and crack, but dry enough that mold doesn't grow and moisture doesn't destroy the cross-overs.

The oldest speakers I have are a pair of Polk Model 10 A's. 1979. Excellent speakers for their day. They still sound like they did in 1979. 
I'm skeptical of new technology that promises "better" sound. No one really knows how they'll hold up (or if the new company will survive to service your drivers down the road). There have been countless innovations that have turned into boat anchors. I stick with what is tried and true. 

I've had my Dunlavy SC V, IV, III, I and Cantatas for over 20 years. Had to replace 3 or 4 drivers along the way. Outlasted all my electronic upgrades through the years. Have never once had the urge to replace the speakers.

Oh yes, and I should include the subwoofer column in an SC IV enclosure in the two channel system.

Recently bought a pair of used PBN Montana XPS speakers. From the serial #s, they look to be from 2006. They are near flawless in every way and I don't see why they wouldn't last for a few more decades. Of course all speakers are built to different quality standards.