Like any 'old' amp (or anything for that matter), the key to making sure it runs well and reliably is to service them every now and then.
Replacing faulty tubes, cleaning jacks, pots and switches with contact cleaner are simple DYI items. Even cleaning the cabinet every now and then is good practice. For more complicated issues (such as electronics) are best left to qualifying technicians with know-how (unless you really know what you're doing)
Cars, trucks and other machinery are serviced.... it's no different for an amp or other musical equipment requiring basic maintenance or repair. I think some people get hung up because it can cost money for repairs but so does anything else that needs repair.
If you want to make sure everything is in top working order, take it to a technician that knows what they are doing and ask to give the amp a 'once over' and check everything to make sure it's running to specification.
I think we believe an amp 'should last forever'........ it will if it's looked after.
-B