The cost of new nuclear powerplants is obviously not a relevant argument for turning existing nuclear power plants off prior to the end of their design life.

Indeed no. The argument about shutting them down is about safety.

A completely objective assessment of the safety of Europe's nuclear plants is not possible, because you can't buy insurance against a Fukushima type disaster. i.e. there is no rational way to assess whether the risks are justified by the economic benefit.

The reason new nukes are completely unaffordable, and will not be built without sovereign strategic decisions and sovereign financing (whatever fig-leaves may be employed) is that providing a high level of safety is horribly expensive. Pro-nuclear people no doubt believe that the safety standards that are now required are absurdly excessive. But this is demonstrably in the domain of belief.

_{It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II}