Speaker Life

If it is a high end speaker probably longer than you live yourself - given the fact of proper use in ANY respect (no overloads whatsoever, no direct sunlight, no bumps whatsoever, no clipping amp etc.)

A speaker from the 80's will suffer from foam surround rot. This is deteriation of the foam 'ring' which connects the cone to the frame. Second item of concern would be the caps. Electrolitic caps may loose storage capability or take a memory set or even leak over time. I assume you are talking of decades here and not years.

Yes I am talking many years. My current speakers are 25 years old (have changed everything else more than once) and I am concerned about transistors and capacitors in the crossovers beginning to operate outside of specs.
I am now looking at upgrading with used speakers and wondering about the wisdom of getting very expensive when they were first bought and now 10 plus years old or more, and therefore now cheap. Comparing this to moderately expensive and say now 2 years old such as Sophia's Watt Puppies etc.