We have needed to build nuclear power since the day it was invented. The amount of fossil fueled power generation is indisputable proof of that.

The only way we don't need nuclear power is if we have some other way of producing the energy we need without dooming the planet, and obviously we do not as we're still burning billions of tons of coal every year. Not to mention gas.

We could have made power generation fully fossil free many decades ago if we actually took climate change seriously. It would have been fine. Sure fossil fuels may have been cheaper, but if you include the damage they're already causing and will cause in the future you could multiply the cost of nuclear by orders of magnitude and it would still come out on top. Plus higher investment would lead to faster improvement in the technology, mass production of components and lower cost etc.