There's reasons to think that a Keynesian program (even one which focused on war, not building useful infrastructure) could have ended up fixing a truly out-of-kilter economy.

But it only fixes medium term (~10 year horizon) problems. Once the monetary / investment system is fixed, Germany would have stalled. It suffered from massive brain drain (if you literally have Einstein, making him flee isn't a great idea). And totalitarianism isn't great for creating an innovative workforce, even if you can keep your population from fleeing.