AFAIK, the Union/FDP government reversed that decision, made new contract with the plants, changed their opinion again after Fukushima and we now have to pay breach-of-contract fines to the nuclear power plants.
If twenty years ago it was decided in law that X would shut down about now, and X is shutting down now, then I don't see a reason to not say that the decision to shut down X now comes from a law twenty years ago.
Mine is that the decision to stop was taken by a government with a parliamentary majority in general, but narrow popular backing in this specific case. So the law at risk of revision if the right/wrong parties won an election. Some politicians thought revising it might be a good campaign issue.
The reacter lifetimes were extended after such an election, and I think it was a first step. If that had gone well, one of the parties in the coalition would've proposed revising the law before the next election. But it did not go well: "Fukushima ändert alles", said Merkel, and I think she was right. From that point on, the law aligned well with a broad majority of voters. Noone proposed a revision as a campaign issue after that point.