30 yr old speakers- time to replace drivers?

Crossovers can last waaayyy more than 15 years. Hell I just sold a 50 year old sansui 9500 that would put the majority of integrated amps under 5k to shame. I know it’s not a speaker. Original caps other than a few small input caps replaced. If there’s nothing that sounds “off” I wouldn’t mess with it. 
 

upstairs system has original Paradigm Studio Monitors. Same thing 30 years old sound awesome, lacking no where compared to my main system downstairs. 

That said I am also with others if they need some work I would not put much into them if anything.

Degradation in SQ with rubber surrounds is brand and design dependent. I do not know your speaker.  In a vacation home in Vermont I have a vintage system used for background music, with 1981 DCM TimeWindows, with rubber surrounds.  It has been published that the drivers in this brand and model have longevity. There is no change in durometer of the rubber or SQ of the speakers from my experience. However, they sound like 1981 vintage speakers, through good ones for their day.  If needed, I would not invest any money in them even though they have sentimental value. There has been so much improvement in speaker design and SQ at all price points that I would go new. 

It’s nice to know the cut off for speaker value is 30 years ?? Pompous load of Doodoo, I think the First consideration should be the quality of the product. A Hi end excellent speaker of the past will still put many to shame today. I’ve people in my group with the likes of Tekton to B&W, Sonus Faber, and more, they’re happy I’m happy. My Infinity IRS system is spectacular and hands down considered the cream of the crop in that group. Granted you need the space having that, next was the total expense… middle of group !  Refurbished I’m good for years to come. It’s a hobby you can rebuild, modify, great sound doesn’t have to equate to great money. 

If they sound good and the surrounds are fine, use them!