What's the lifespan of an solar farm with equal output to a nuclear plant? We're about to approve nukes for 80 year life spans.
Nuclear has super low operating cost and a long lifespan. The only real economic downside is how flipping expense they are to build and a major reason for that is we don't have much experience building them (because they've lasted so many decades).
You're also assuming that solar/wind will be faster when we don't have any real solutions for using those sources as a baseload. Storage and distribution is nowhere near being able to handle such demand. The cost of revamping our grid once those technologies do reach maturity is going to be staggering aswell.
Solar and wind are promising but they are still a __long__ way from being proven enough to gamble an entire energy plan on.
And that scaling renewables is largely unexplored, and very likely similarly expensive problem.