Lifespan of amps and preamps?

There is no hard and fast rule on longevity.  My system utilizes a midrange compression driver that is about 80 years old; it is one of the very best drivers that I've heard of any kind.  My amps, built about 8 years ago, have a lot of very old components in it; the output transformers are around 70 years old.

A friend recent bought an old Western electric 124 amp that was not reconditioned--all original parts, and the tubes, were also old and quite rare--WE 274, WE 348 and WE 350.  The amp is in perfect working order.  Western Electric built stuff for commercial use and with a cost-is-no-object philosophy; the company leased their gear so they built the stuff to last.

Tube technology is very mature and even very old stuff in good condition will be competitive with modern versions.  Also, most classic tube gear can be easily repaired and replacement parts are not hard to find.  

Solid state is a bit different.  That technology is less mature so there are technological changes (e.g., Class D operation) that have come to the fore in the last twenty years.  Also, it can be harder to repair solid state gear because certain parts can become hard to find, such as transistors and integrated circuits.