I may agree with your conclusion that old plants are safe enough (or at least take a deep dive study to see if their expected externality is worse than whatever would replace them). However:

> the worst disaster to ever happen without any external factors

The problem is external factors happen. You can’t just raise your hands up and say “wasn’t my fault,” when they do. A tsunami washing over a solar farm would be a lot safer than what happened at Fukushima.