The entire U-boat campaign, while exciting and romantic, was nowhere near as great a threat as it ever seemed to be. In its best month the U-boats never even got close to sinking as much tonnage as was estimated to be required to severely cripple Britain's home economy.
The what ifs are economic and political. Build more U-boats for the start of the war? Give them more support to increase their effectiveness? Get the XXI out earlier than expected? That all could have made life much harder on the Brits.
I don't think it would have changed the outcome, especially not with a guy like Hitler making strategic choices, but it could easily have extended the war and changed the complexion of the history we know so well, delaying things like D-Day.
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