How long should high quality speakers last?

The serial numbers on my Altec 604Cs and on my 604Ds are 1956 and 1958 respectively. I have had them for 45 years.

I rotate them once a year, and have them serviced every 15 years.

The crossovers are Mastering Lab. The caps have been replaced. 

@mkaiser 

Did I say “sound the same as new”? 

I did not. 

However, I obviously would not still be using them if I was not getting anything less than outstanding performance from them. 

The rest of your statement makes no sense to me. 

  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,