How long do speakers last?

If the drivers have foam surrounds, there's a chance the foam will deteriorate and fall apart in 15-20 years.  They can sometimes be refoamed depending on availability of appropriate replacement foam surrounds.

I don't know if the VR33s have drivers with foam surrounds.  If the drivers are rubber not foam, then they could last much longer.

Beyond 20 years, the capacitors in the crossover may require replacement.

nice review here

My speaker's drivers were made in 1958.

I have re-coned the 15" woofers 3 times over the years

The tweeter horns and mid horn are both impregnated linen, essentially indestructable, the voice coils do not burn out

The crossover is elements buried in a tar filled can. I found someone who makes modern crossovers for many speakers, he knew mine, and said if working, don't replace them.

I have had to replace my L-Pads, 2 each side, I neeeded to find 16 ohm.

I also have a set of spare drivers, for the next generation.

I moved my innards to new enclosures, but yours are special with sand filled cavities, rear tweeter, ... The grille cloth may be weak from age, is not normally removable, but there must be info on the web.

If the speakers, both or one, was located near a heat outlet, that side drivers, especially the woofer lower near heat vents.

I would contact the maker and perhaps buy spare drivers now.

Upgrade ideas must be out there, perhaps you will be modifying the crossover

Gee, let's see.  My Tympani I-C speakers have been sounding better than any other speaker since 1974, so that would be 49 years for mine with no repairs, etc.

Now, I am sure their new ones sound even better, so if you want a "forever" speaker, at least TRY a pair IN YOUR ROOM with good hardware.

Cheers!

This comes up from time to time.  Capacitors go over time; 15-25 years.  The other important thing is consistent humidity.  Too wet and stuff rots, rusts and shorts.  Too dry and the enclosures can warp and come unglued, and the driver cones can deteriorate more rapidly, based on their composition.

A lifetime and then you can ship the drivers to Bill Legal at Miller Sound in PA to fix them.