That has little to do with the decommissioning itself, and more with the Governments stupidity and ignorance. The decommissioning is only one part of that screw-up.
Germany wants (and the whole world should):
* enough energy
* carbon-free
* nuclear-waste-free
On order to do that, you need massive investments in renewable energy and storage.
Instead, they found a way to have:
* large investment in solar energy, without protectionist measures, leading to the German solar industry dying an creating demand that lead to cheap China solar (overly simplified - feel free to elaborate in your answers)
* laws that limit the building of wind power significantly
* failure to build a power line from the north (offshore windparks) to the south (high power demands, less power resources) which currently leads to insane situations with the neighbors
* first extending the nuclear power plants permits, shortly afterwards deciding to shut down, which lead to Germany having to pay compensations to the companies (in addition to a completely botched nuclear fuel tax, that they had to pay back after it was found invalid in court)
* subsidies for dirty coal plants over decades (300 billion was a number floating around) and another I think 50 billions to help the regions which rely on the coal business after the coal shutdown
Politicians deal in compromise. They see different interests and they try to find a middle ground between those two. They do that with a pretty heavy disregard of science. Scientist are just another interest group.
Scientists: If we don't cut CO2 but this much until this date, the effects of climate change will become this nasty.
Industry: If we do that, that will cost us X moneys and endanger Y jobs and we will make Z less profit.
Government: OK. let's meet halfway.
Earth: LOL