Shutting down at the intended end of life is a third decision point.
New renewables are approaching the marginal running cost of nuclear that is still within their intended life span.
It would need to be shown that an expensive refurb is better than running it down efficiently while building out new renewables as far as bang for buck in getting off imported gas.
in belgium case the choice is rather nuclear or new gas plants from engie. Why do you think engie wants them shut?
And importing gas famously has zero known serious externalities, as vividly demonstrated in Europe and the Gulf at the moment.