1. It tied down resources that would have allowed Nazi Germany to hold the Russians off. Freeper LS presents an analysis in his book America's Victories that shows the Battle of Kursk would almost certainly have gone to the Germans if they had been able to devote their Reich Defense resources to the Eastern Front. Note that about 1 million men served in the Luftwaffe's anti-aircraft gun units alone. What difference could a million troops have made on the steppes of Russia, or in Normandy in the Summer of '44? In a sense, there were two fronts in Europe years before D-Day, and the Western Front was in the skies above the Reich.
2. Damage to Germany's production capability was often far underestimated because German efforts to increase production made up for the damage. In his last book, The Bomber War Robin Neillands backs that up, and notes that in the early years of the war, when England and America were already running their factories around the clock, Germans were still working one ten hour shift. As the bombing offensive destoyed production capability, they went to round the clock and brought in slave labor.
3. The P-51 destroyed the Luftwaffe. Most importantly, it brought them to a point where they had plenty of planes, but their pilots were dying so quickly that replacements got little or no training. No strategic bombing, no P-51. No P-51, no deep excursions into Germany, leaving the Luftwaffe able to provide tactical support on the Western Front after D-Day, or (as previously mentioned) in the East where it would have made all the difference.
Now, that said, the idea that the bomber could bring a nation to its knees on its own was not feasible...but the bomber did win the war in Europe, just as surely as the ASW forces did.