The problem with old aircraft is metal fatigue, and it is an unavoidable fact that old engines occasionally give up, plus that spare parts might have been wrongly installed or below-par items have been used as replacement items. Non-TSO-ed modifications might have been added without getting OKed from FAA.
Add to that in-flight fires, pilot errors and instrument malfunctions leading to flight terminations before the planned return to terra firma.
Remember a flight in a military DC-3 (it had had a long and varied life even then) years ago, that ended with aborted flight and a quick return to the airport, where every kind of emergency vehicle we had was following us along the runway, till we stopped and they soon swarmed around the aircraft, which just had blown a plug. Sobering it was, though.