You would have a safety margin because it's part of the design: the solar+wind super grid supplies power most of the time, but you also have quick-starting gas turbine plants for filling in the rare continental-scale lulls. These would be local and immune to grid damage, which can be repaired relatively quickly.
The planning and control of the super grid wouldn't be simple, but then it isn't for nuclear either.
So you are advocating not only to build massive amount of wind and solar stations everywhere, massive because they simply have a low GWh output compared to their theoretical peak power, and a massive and complex grid to connect all that, but on top of it you want to install and maintain lots of "emergency power plants" so that local peak demands or failures of the grid can be dealt with?
Because the you would basically need the same amount of capacity of power plants installed as we have now, and while not operating 24/7, you have still to maintain them.
Sounds a bit nonsensical to me, if we need that amount just for backup purposes. Mind you, building safer, modern nukes, and getting LFTR's and traveling-wave ready for "prime time" to replace the old ones as soon as possible would be a much cheaper option (materials, building work, space needed) in the long run.
Somehow your idea doesn't sound like as if it was thought through.
But then, at least in Germany, the funding of science and development of new nuke technology was virtually stopped long ago, with the decision to exit nuke tech completely putting the final nail in the coffin. After all, who wants to spend lots of money on research and development on something that is 100% guaranteed to never get build at all?
It's easy to point at old technology and their flaws, running around and telling people that it is that way for so long already and nothing new comes along, when you effectively blocked exactly that to begin with. Same goes for the situation with spent fuel. Blocking all plans to get fast breeders build, actively forcing the operators to vitrify the waste to make it unusable for reprocessing, but then pointing at the problem of terminal storage. It's hypocrisy beyond belief.
Greetings,
Chris