I think the USAAF daylight bomber raids did have the effect of wearing down the Luftwaffe's strength. Gradually, their top pilots and aces began to be killed off by the gunners and escort fighters. As their force started to dwindle, they were uneffective in attacking the Allied infantry, there wasn't enough fighters on hand to try and stop D-Day. Germany constantly under attack had little time to think about making their own long range bombers to attack England, all their aircraft production went mainly into fighters to counter the bombers, which left them only the V-1 rockets to attack long range.
The daylight raids helped the Allies gain air superiority, and that was one of the biggest effects it had on the war effort, besides destroying many Axis factories along the way. It came at a great cost and the price was high, but in the end it worked. Could it have been handled better? Yes, for one making sure the bombers always had fighters along to decrease losses.