Tube preamplifier lifespan

the owner of psaudio told me that they do, lifespan of only 20 years

Parts quality likely biggest variable, amount of heat generated also important, heat enemy of parts reliability/longevity. My pre uses DHT so relatively low heat, has two chassis, one for power supply, other signal path. 

the thing that confuses me is that i've heard that Mcintosh preamps, using full vacuum tubes last forever. where the BHK preamp only uses two small tubes on the input and mosfets on the output

Perhaps the generalization depends on the brands or the initial quality. 

i'm concerned, since i traded in my 20 year old Bryston preamp (still under warranty) for my 'retirement' system