How long do good speakers last?

I still have my Pioneer CS 99a speakers from 1978 that are still in great condition. Surrounds are cloth so never need replaced. They're in the barn along with my Cerwin Vegas VS 150's I bought new in 1997. I did have to recone the foam on the Vega woofers last year, but both pair still play flawlessly. Keep in mind these speakers are in a dirty, nasty, farm barn with critters, dust and filth. Haven't even been dusted off in 5 years. I would think technology would have them last even longer.

I have a pair of Klipsch La Scala, which were made in 1981 and  a pair  of K horn Speakerlab, which were made in 1971 for more than ten years and a pair of JBL K2 9000, made in 1990, which I bought 3 years ago. All of them are working with no issue. I thing they sound perfectly, but I all way wonder if how much their sound quality reduce compare to the new ones?  

They'll last until you find something you think you'll like better.  All of a sudden there will be something wrong with them.