> The German decision to phase out nuclear power was a very big and very costly mistake

It's time to stop talking about it. It's done. Unless the stopped plants can be restarted (which I'd support) this is completely useless. It doesn't mean anything.

> How much further investments in solar, wind and storage will decrease the need for gas?

The upper bound today (keep in mind battery tech gets cheaper all the time) it would cost $5tn to power Germany on batteries for 6 months. https://news.ycombinator.com/item?id=45446112

You don't need to run Germany on batteries for 6 months. Even 1 month is more than plenty.