I'll make kind of absurd argument, but how is nuclear not competitive with dying? You could always tax the shit out of fossil fuels and they won't be competitive. You could also make other laws that punish fossil fuel usage with death penalty since you could argue its a crime against humanity. I mean, law is an abstract construct. This is a live or death situation. Even if we need to abandon 30 years of economic growth for the sake of surviving, then maybe it's what we need to do.
We have technically been able to do this as a species ever since global warming was discovered yet even in the face of an extinction level event we apparently still can't make ourselves do it. Fossil fuel lobbies are just too powerful. Carbon taxes are mostly weak or non existent. It's the species level way of saying "why don't you just give up smoking?". Not that easy.
Yet it is apparently possible to make solar + wind + storage + demand shifting energy cost competitive even without subsidies and very competitive with subsidies. The fossil fuel industry hasn't managed to shut it down. Species level nicorette patches.
One day we'll be able to tax the shit out of fossil fuels - once it is a nearly dead industry without much political muscle. By then it probably won't matter, though.
It's pointless posturing: following, not leading.
They'll gradually and then suddenly ratcheted up as the fossil fuel industry loses political power in various countries. It'll be far too little and too late to mitigate the worst effects of global warming though.
Your suggested laws ain't coming.
Whilst I agree with your broad point, climate change is a systems problem and requires systems thinking and systemic change.
If you make nuclear available, but it is not competitive with fossil fuel energy, then we will continue to use fossil fuel energy. A relatively small number of oil company executives with a profit motive are going to make this decision for everyone, directly and indirectly.
'Doing stuff', as you say, involves governments of the people heavily regulating the most powerful corporations on the planet to ensure they can't make this decision in the wrong direction.
Government action in this direction hasn't happened for the last 30 years, and doesn't look close to happening in the present.
As such, it's reasonable for climate change activists to make plans to meet the market where it's at and prioritise competitive renewable products above nuclear.