For an amp of 1978 that has seen regular use, the dominant factor will be how hot it runs. A high-bias amp will dry out the capacitors much quicker than one that runs cool. They will degrade slowly, but then can short out all of a sudden.
But there are many other factors that also contribute to longevity, like the quality of the caps, their ratings and type, how much voltage stress they see in operation, where they are located in the chassis, chassis cooling, if the amp wasn't used for long periods of time, the available space around the amp when it is used, if something was stacked on top of it, the typical ambient temperature in the room, the humidity, etc. This is why generalizations lead to unpredictability.
Arthur
But there are many other factors that also contribute to longevity, like the quality of the caps, their ratings and type, how much voltage stress they see in operation, where they are located in the chassis, chassis cooling, if the amp wasn't used for long periods of time, the available space around the amp when it is used, if something was stacked on top of it, the typical ambient temperature in the room, the humidity, etc. This is why generalizations lead to unpredictability.
Arthur