Yes, even if D-Day was lost, the allies would have won WWII, but the fighting would be much more difficult, and the war would have raged for much longer. Additionally, the loss of human life would have been much greater. It would take a longer time for the US and the British to break Nazi lines in Western Europe, and the Russian advances from the East would have been less successful as the Nazi forces would not have been retreating as heavily. However, the world was essentially united against the German power, so in the end, the allies would have emerged successfully.
We can play the game of what if all day long and at the end of the day it does not really matter. D-Day was an important victory that really changed the tide of the war. Had this battle been lost then who knows what would have happened. Like I said, it's all a game of what if.
The Allies winning at Normandy was very important. However, I don't think it was the deciding factor in the war. The Soviet Union fighting off the Germans in Russia was more important in defeating the Nazis. Had the Allies lost the battle in Normandy, they still would have been able to win the war, thanks to the Soviets.
The battle of D-Day was a critical turning point for the allies during WWII. It this battle was lost then the war may not have been won. D-Day was the largest amphibious attack in history and the results liberated Western Europe. The Battle of Normandy lasted 3 months in the summer of 1944 and by the following year the Germans were defeated. The battle was a turning point for the war.