ggoebel:
There is no one answer to your concern. It would be expected that the cones and speaker surrounds will degenerate over time but that depends on what they were made of, what kind of climate they were used in, how much use (and abuse) they experienced, etc. I expect that you will have to have the speakers inspected by a genuine speaker expert to get a valid answer.
I had a set of Sansui speakers (for a 4-channel set-up) that were the best Sansui made in 1971 (two front and two surround speakers). They were not only beautiful but they sounded great. My ex- got them in the divorce settlement (not because she had any idea of what they were but because she wanted to "punish" me). I started looking around for replacements and quickly found that there weren't any available and those that were seemed to have suffered more than my (ex-) speakers. The combination of scarcity, money and time led me to buy different speakers. Since speakers are THE most vital part of sound reproduction (IMHO), I didn't want to make an expensive, time-consuming mistake. So, I am now using Magnepan 1.6s in my music room and Vandersteen speakers in my HT. Not the same but pretty amazing nevertheless.
Sorry I can't be of more help...