Things that can go wrong with vintage speakers:
1. The foam surrounds rot.
2. The wood veneer dries out or the paint/vinyl gets scratched.
3. Crossover components go bad.
4. You over- or under- power them and blow a driver.
5. Your kid/pet/spouse/drunken friend sticks a pencil/screwdriver/knife/finger/toe through a cone or dust cap.
6. Your angry girlfriend/spouse/partner smashes them with an axe or hammer.
7. Your angry girlfriend/spouse/partner throws them out of a second floor window.
8. They get stolen.
9. They burn up in a house fire.
10. You run over them with your car.

Numbers 1–5 are fixable, and you can return them to new or better than new performance. This assumes they were good speakers to begin with.

Numbers 6–10 mean the death of the speaker, whatever its age.

I have some vintage JBLs that are 35+ years old that have never had a lick of work done to them. They look as new and still sound great. I have some 15 year old speakers that had foam rot, pushed in Ti domes, and damaged veneer. A little work and some professional refoams put 'em right, and they sound fantastic still.

Many of the best deals are on 5–15 year old speakers.