Lifespan of a quality solid state amplifier?

Agree. To build powerful SS amp that would last for 40 years is challenge. Bryston deserves respect.

Any gear if used serves much longer that the one that used occasionally.

Just checked 50y.o. HH Scott 222c filter caps and both only lost 16% of capacity.
That makes me think of weather to replace the original or keep it. Replacing the original might require some chassis cutting for the new fitting and near $50 per piece.