I recently did some studying up on capacitor life expectancy. Traditionally, electrolytic caps are the first thing to go, after tubes. But like tubes, they can last a long time if they are not abused.
For the caps, it's heat that kills. Caps are specified for a certain lifetime at rated voltage, temperature, and ripple current. Most of our caps are specified for 2000 or 3000 hours - sounds pretty bad, doesn't it? :^) But that's at 85 degrees C - that's 185 F, which is plenty hot enough to curdle your Hollandaise. At 45C (113F) that's 32,000 hours, or 8 hours a day for 10 years. I have not made enough measurements to make a claim about the under-hood temperatures, but the caps are easily felt to be cooler that the transformer core, and even our hottest transformers are less than 140F (65C). I have designed for 60C under the hood but I think - at least for the more recent amps with cooling slots - 45C is a good guess.
The most serious long-life electrolytic caps are rated at 125C and sealed in welded stainless steel cases. They'll supposedly last a million hours at 45C - and for only $200 each!
Those are of course the predicted lifetimes based on engineering studies. Those who read the papers might have heard about the new Boeing 787 and it's battery problems. The real world does not always work exactly like the engineering models.