The negative externalities of fossil fuels justified some level of relative subsidies. Either by taxing pollution or direct subsidies of nuclear power.
Even ignoring global warming smog and acid rain have been known issues for a very long time. On top of this many countries lacked domestic supplies of fossil fuels.
For making nuclear viable, "ultra cheap" here would have to be a negative price.
The similarity of the problems with renewables is only that they benefit from storage. Without any similarity on the economics.