How long do speakers last?

@allenf1963 

It's a simple chemical fact that electrolytic capacitors degrade over time.

"When life expectancies exceed 15 years the expected life of the capacitor should be limited to 15 years mainly due to the sealing materials deteriorating over time." -  Illinois Capacitor

"The quality of the oxide layer can deteriorate during storage without externally applied voltage, especially at higher temperatures. Since in this case there is no leakage current and as a result, the oxide layer will not regenerate. This leads to a higher leakage current flow when a voltage applied after prolonged storage." - Würth Elektronik

NOT playing may be more detrimental due the above aging effect depending on the initial construction and chemistry.

The quality of capacitors today is several generations better than 40 years ago. When done properly on older speakers, the improvement is well worth the investment. 

Additionally, changing components [electronics and interconnects] affects the sound. As do room, humidity and CBLF aging. So unless you still live in the same place with exactly the same system and hearing as 40 years ago, claiming identical sonics is specious.

AR-2ax speakers here from 1966 still going strong and sweet!  This was before AR fitted these with foam surrounds, so the original butyl rubber surrounds are still perfect.  One mid range unit was replaced in the mid-70's ($9) with OEM;  one dome tweeter replaced in 2015 (with perfect aftermarket unit from Parts-Express - $20);  and 4 level potentiometers replaced with L-Pads ($30).  Crossover caps are still fine. Woofer foam surrounds can be problematic after 10-15 years.

i have 4 bose 901 series 2 speakers i bought new back in the day! in 2000 my phase linear 400 power amp fried 3 of them! the 4th, is still original and is still kicking it!!

Speakers can last a lifetime. Recone, recap, etc..

Without having to do anything to them they last exactly 20 years, 275 days, and 30 seconds.