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.