Not building more nuclear in the seventies probably was a bad decision. But today it is not at all clear that nuclear is the cheapest solution for carbon-free energy. Neither is it clear whether we can scale nuclear quickly enough.
If I waited for the clearest, best solution to become apparent to me every time I went to get something done I would still be deciding what food to buy at the grocery store. Sometimes it makes sense to run with a "good enough" solution until something better can be found because the alternative is doing the shitty thing you've always done, or worse, doing nothing at all.
Renewables could also be the "good enough" solution, at least until 70% penetration or so where you want to start building storage. Starting with those has the benefit that you don't need to sink a couple of billions into a big construction site and then wait 10-15 years before the first kWh comes out. That buys you more time to figure out what the actual best solution is.