We stopped the Japanese momentum at Midway....if we didn't stop it there, we'd have stopped it somewhere else. Our manufacturing and industry would've gone even higher than it actually did if we had to carry the war into 1946 or 1947.
A successful D-Day probably saved millions of German and Japanese lives as well. If our invasion had failed, we'd have re-grouped in England, the B-29's would've firebombed Germany into the Stone Age, and we would've carpet-nuked our way to Berlin. Japan would've ceased to be a race (Bull Halsey once said the only place the Japanese language would've been spoken "was in Hell").
I have a memento of Japan's battle plan from December, 1942.
It is a Japanese war bond with clippable coupons. The coupons are redeemable for 50 yen, with the final payout in December, 1959.