Dome tweeters with ferrofluid, foam surrounds, rubber surrounds, damping goop covered boards, capacitors, even resistors (esp if running hot) and for all I know some enamel on inductor wires, all wear out.
Oh and wood changes over time, and particle board can degrade over time.
I have several pairs here, featuring issues with some of the above at one point or another.
My oldest pair is circa 1970, using 1960s tech. Hence no rubber, foam, ferrofluid, etc.
So drivers appear to be good as new. Crossover replaced entirely with new components (same design).
The chipboard of the case is a bit iffy in one or two places (though still maintaining structural integrity so far with a bit of help and injected glue).
At some point I’ll have to give up on them though.
Apologies for not directly answering your questions, it’s late here