There arn’t really a lot of fossile fuel plants that are decomissioned before the end of their lifespan, so this is more of an hypothetical solution for maybe sometimes in the future, all the while we continue burning fossile fuels. So basically it is an distraction from doing what is needed now.

And once a fossile fuel plant reaches the end of its lifespan the only usable things that are still there and can be reused is the electricity network connection. So those sites are good locations for cheap grid level battery storage (and maybe novel geothermal), but not small nuclear reactors that still need turbines and cooling towers etc.