I take my cue from Winston Churchill, who ought to know what he was talking about.
He said the only thing that really scared him during the war was the U-boat threat. It really is the only thing that could've knocked the Brits out of the war, and when the threat was defeated, victory in Europe was absolutely assured.
If the U-boats had won in the Atlantic, Great Britain was lost, and with her, the war.
But when the convoys became unstoppable, so was the Western Allies' war machine, and the Reich was simply doomed.