A hypothetical thousand Type XXI submarines available in 1939 probably would have caused severe problems for the Allies. That concession does not remove the practical limits of the scenario. Germany lacked the early-war basing facilities it possessed later when the type actually appeared. Unless the boats arrived by magic, their construction would also have been visible to other countries. Those countries would have reacted during the buildup, so the scenario cannot assume that Germany alone changed its preparations.
Notable Point · 54:19-55:04
A thousand Type XXI boats in 1939 would still need bases
The Type XXI counterfactual concedes severe Allied difficulties while accounting for early-war basing limits and foreign reactions to a visible building programme.