Climate change is a (existential?) threat to humanity. There is only so much renewable energy generation capacity that can be constructed in any given year. If you shutdown a nuclear power plant, the demand it was servicing does not magically go away. So you need to something to meet that demand: that may be renewable, but if not enough can be constructed, then it could also mean running fossil fuels more.
So if you want to actually treat climate change as an important problem, why would you shutdown non-emitting source of power that you already have?
Keep the existing nuclear plants, shutdown the fossil fuel-based generators, and use the renewable power you're building to offset the retirement of the latter.
Once all your carbon-emitting generators are retired then consider shutting down the non-emitting nuclear generators.
At no point in the above process are new nuclear plants built.