Why are grand coalitions so toxic to the SPD electorally?
The general trend globally at the moment seems to be that the junior partner in any coalition takes a beating at the next election. Its especially the case for grand coalitions - the SPD can't credibly stand as any form of alternative government if they have to also defend their record in government. Happened in the Netherlands as well: the PvdA basically lost a significant amount of even their core support and aren't even the top left party in the Netherlands, which is arguably bad for the left generally since sadly its much harder for harder left parties to get enough support in order to start forming government. This is why, in the absence of a World War or some other serious crisis that requires the formation of a government like that, generally it should be avoided. It all goes back to 2013 and the stupid decision of the SPD to not even talk to Die Linke about forming a government of the left with the Greens and instead instantly going back to the Grand Coalition - they'd have probably lost support as well if they'd done that but at least they'd have led a government for a few years, rather than been under Merkel for four and then losing more support than she did...
