How long should high quality speakers last?

  Why wouldn't speakers last for a very long time? Apart from replacing the foam surrounds and capacitors, or perhaps re-magnetize the magnets, I haven't and I don't know why you would need to. 

it’s a function of materials used and when they deteriorate. Foam surrounds have a 20 year(?) life. Anyone that recones drivers should be able to answer.

My experiemce….I heva 20 year old thiel 2.4’s that as afar as I can tell sound as good as the did 20 years ago. They were 4500 at the time, which I would call resonanbly high end. Generally their reviews are mostly positive.

Upgrade? I see no reason to, An upgrade would likely be expensive, thus costing me 16k or more. Seems silly and I’d probably be lucky to get away for 16K for the pair,

I had Bill at Great Plains Audio rebuild my Altec Lansing VOT 416-8 woofers two years ago. He reconed them with OEM parts and remagnetized the 2.4 lb alnico magnets. They were rated at 12,000 Gauss when new. If you drive the speakers hard the magnets will demagnetize.. These speakers were from the 50's and I still love their sound and prefer my Altecs to my Theil CS3.5's

When Alnico drivers have partially demagnetized you will notice a difference in efficiency and a noticeable loss in the higher frequencies. An alnico driver in need of recharging sounds kind of dull, and doesn't have as much transient jump. Basically, it sounds dull.

Speaker magnets can be regaussed by Bill at Great Plains Audio in Oklahoma City.

High quality speakers should literally last a lifetime or more provided: (1) the user does not damage them, and (2) they're kept in a suitable environment free from extreme temperatures and moisture (i.e., indoors).  I have a set of Klipschorns that are pushing 50 years of age and they are still performing as expected.

YMMV but I purchased a pair of ADS L810 series2 in 1978. Used them sparingly until about 6 or 7 years ago and passed them down to my little brother. He is still enjoying them today. They have survived many moves but still play like new. All original parts.

Of course, all of these are anecdotal and speaker design, quality and your luck will determine how it turns out.