Even if it wasn't the case that PV's could have replaced fossil fuels back then because they weren't advanced enough nuclear was more than capable of doing so and readily available. Indeed investing in various different types of reactors might very well have produced a similar curve of progress and finding ways to make them less dangerous and requiring less regulation all of which would have sped up deployment and reduced the cost.

We will never know, we just know now that its cheaper to do solar and power storage than nuclear.