But then we have the second issue, which is that we don't want the alternatives to be significantly more expensive than the status quo, and we may want to take some steps to ensure that.
If you tax carbon and so people switch to renewables and batteries and figure out how to make it scale efficiently, everything is fine. If not, people would then turn to nuclear because it would still be less than paying the carbon tax.
But if we wait to start building nuclear plants until after we've already established whether or not renewables can handle the entire grid, and then it turns out they can't, that could be problematic, because then you've waited too long.
It's a hedge. If we fix the nuclear regulations to remove the most egregious of the cost-wasting portions and start building nuclear plants now, and then we don't need them because renewables fully succeed, energy will cost modestly more than it would have. Or the investors in nuclear would lose money in that case. Whereas if we don't, and then we do need them, energy would cost dramatically more than it would have because of the shortfall. And as a result people would be inclined to pay the carbon tax, or repeal it, and then we get catastrophic climate change.
When "need it and not have it" is literally the end of the world, better to have it and not need it. That seems like a worthwhile hedge.