Economic downturn hits. Company or rich family decides to cut costs. Plane(s) get sold. Flight department gone, replaced by netjets/flexjet or commercial for their travel. Or that same thing could easily happen even when not in an economic downturn. The chances of a corporate flight department shutting down over 30 years is higher than a 121 carrier shutting down. Or, as you said, if they don’t like you, getting fired is a thing. Not as easy to get fired at a union airline. No pilot job is stable except maybe the military (even then not always the case). Airlines seem a bit more stable than corporate though.