So if anything, they're (partly) responsible for keeping a dead/dying industry on life support. Can't blame them for not wanting thousands of people to lose their jobs, but market forces were definitely not favorable to American automakers at the time.
Then again, American automakers seem to be figuring things out. The problem with counterfactuals is that you can never actually observe them...
Also, German unions are less antagonistic to management than American unions. The UAW took over the Big Three in the 50s and 60s, and they immediately went from extremely successful, to getting to the brink of bankruptcy.
It might be that unions are the reason why German car companies are doing well, but there are definitely other factors at play too.
The same could be said in the other direction. Fundamentally they are set up differently, and one of the results is that corporate-union relationships are more collaborative in Germany, more combative in USA.