Fission (and likely fusion) ended up being too complicated and expensive to do the job. But renewables appear to be much better situated to push out fossil fuels over the next several decades.
If we want to get serious right now about eliminating fossil fuels, the fact is the answer is a mix of renewables and nuclear.
At this point it's a race against the clock and we're losing it badly. If we can't transition to it before it's too late, then that we could do it in theory isn't worth a whole lot.