What key advantage did the Allies gain after sinking more U-boats than the Germans could replace by 1943?

The key advantage gained by the Allies after sinking more U-boats than the Germans could replace by 1943 was indeed control over the Atlantic Ocean. U-boats were essential to Germany's strategy for disrupting Allied supply lines across the Atlantic. By significantly reducing the number of operational U-boats, the Allies were able to ensure safer passage for their supply convoys, which were crucial for providing troops and equipment to Europe.

This control over the Atlantic allowed the Allies to enhance their logistics and supply chain, which was vital for sustaining military operations. It enabled the successful execution of strategies such as the D-Day invasion and other offensives in Europe, as supplies, troops, and equipment could flow more freely without the looming threat of U-boat attacks. With the Atlantic secured, the Allies could more effectively coordinate their military actions and strengthen their position on the European front. This dominance was a turning point that contributed significantly to their overall success in the war.