How long do good speakers last?

If this has been mentioned before then ignore. Although we are all supposed to believe that things made now are better because of the new materials and technology available that is not always the case imho. I use on a daily basis speakers made in 1978 and 1990 and they both sets are working fine and neither have put a foot wrong during all that time. Of course grill foam disintegrates and has to be replaced but that's all in my experience. Treating the speakers well and not subjecting them to excessive heat, humidity or physical abuse will add to their life. 

It would be interesting to ask the company how they believe their speakers  should last?
 If you do and get a reply, please update us.

Check the wiring.  PVC is a commonly used jacket material. It continues to outgas over decades and the chloride oxides copper. 

Bought a pair of ADS L810 II's in 1978. Used them until about 5 years
ago. Passed them on to my brother and as far as I know, still making sounds. Only issue I ever had was blown tweeter fuses when my worthless step son used them while I was at work,

I have a great pair of Advents from 1972 that had virtually no foam left. Got a couple of OEM woofers (an expensive fix I admit) and now I enjoy the sound I remember from way back when.The cabinets show their age but are still solid. Watch for foam loss and tweeter breakdown and 50-year-old speakers are possible.