Lifespan of components?

How long can we expect a well designed component to last before it fails? I am assuming that eventually everything fails. I am looking at my system, and the oldest pieces here are in the 50 year range. Some components should be easy to replace if they fail- a capacitor, resistor of transistor, but other things not so easy- a transformer or an integrated circuit, and if they fail, their often unobtainable status means its the end if the road unless a parts donor can be found. In some ways I think it could be easier to give a 1960’s component a new lease on life after a failure than a 1980’s IC based wonder that has failed.