>You are right, however, that it is economically somewhat senseless to shut down the nuke plants basically shortly after the subsidies/initial costs paid off, but, well.
How is it more senseless than shutting them down any other time before the end of their useful life? Sunk costs are sunk.
You answer your own question. The amount of useful life they have left that goes unused is an opportunity cost. The earlier they are shut-down the larger that cost (which necessarily must be taken up in providing power through some other means).
Sunk costs are sunk, yes, but at the moment they provide cheaper energy than during most of their previous lifetime. Or put another way, the earlier you shut down a plant, the more you pay per kilowatt-hour generated.
How is it more senseless than shutting them down any other time before the end of their useful life? Sunk costs are sunk.