Why Type XXI boats would still face a growing escort force

Would Type XXI submarines have forced the Allies to negotiate if the war had continued?

Probably not: their arrival would coincide with overwhelming Allied escort numbers and improving anti-submarine weapons and detection.

The Type XXI was formidable and still had to approach convoys to attack them. A war long enough to mature the new boats would also give the Allies time to complete large numbers of delayed or cancelled escorts. The Royal Canadian Navy alone had become a tonnage-heavy force of destroyers, frigates, corvettes, and trawlers, while systems such as improved ASDIC, Hedgehog, and Squid were entering service. A better submarine design would therefore meet a much denser and increasingly capable anti-submarine environment. Any counterfactual that improves German technology must also account for Allied force growth.