the balancing is provided by the fossil plants you're displacing. You don't need any new peaking gas plants, surely?

But the challenge set by asdf was an end to all coal-electricity within five years. So, I took that to mean no balancing with coal either, at least for this extreme-test scenario.

Yes, you're right that it would be cheaper to have some coal for balancing, in a 10-15 year transition. And in reality, that's what's happening: fossil plants just run with lower capacity factors, and might spend much of the year in mothballs.