How long do speakers last?

My Spica TC-50 are 40 this year. Re-capped with all polypropylene matched to original cap values in 2018.

See  ieLogical Audiophilia Redux

I have a pair of Polk Audio Monitor 10's that I bought brand new in 1987 that are in mint condition and sound like they did the day I first hooked them up.

Possible, but not probable. If there are electrolytics in them, they need to be recapped. Replacing mylar caps will increase transparency.

Plus, NO speaker I ever bought sounded like the day I bought them after a play in.

@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.