I think they did improve the cables over the years. The cable on my 1st SX90's rusted out and failed (in the middle of STH at Snowbird!) when they were only a year old. I've never seen any cracking in the plastic on any of my SX's. But still...If I had a knee that was already torn up, the last thing I would do is risk it with old boots that could break at any time. I had a pair of 91E's that I loved...one day on an easy slope with my kids the rear buckle broke. I was actually able to get new replacements from Salomon (even though they were probably 15 years old at that point) but I "got the memo" and bought new boots before the next season.
I am skiing on old boots, but I have replaced them several times with almost-new old stock several times. The pair I'm skiing have only seen about 5 years of active use. I have been very careful to dry them and then keep them dry in storage.
The bolts that hold the cuff to the clog can loosen up too. I had one fall out, fortunately in the locker room and someone noticed. Make sure they are tight.
I don't think I'd use my "old" old boots either.