Transistors also degrade over time but it usually takes longer than 15 years. By the time the unit is 30 years old one can expect a higher noise floor, not only from the transistors but also the resistors. Corrosion can also cause semiconductors to fail for no apparent reason. This is usually a problem associated more with ICs but it can certainly affect discrete devices also.
For now though, since you already have the power supply filter caps replaced, I would expect no worries for quite some time.
For now though, since you already have the power supply filter caps replaced, I would expect no worries for quite some time.