If you accept the adage that war is diplomacy by violent means, the object was to achieve political and economic goals. If Hitler had forced Britain out of the war and into a cold peace, he would have consolidated his rule over Europe, resupplied his forces and would have been in a better position to challenge Stalin and the soviets. In fact it is plausible that a bankrupt , defeated Britain have used its merchant maritime fleet to trade with its former enemy. Germany would have achieved some its objectives and would have been in a better position to complete its aggression against the Soviets. The loss of 300,000 of its best troops and the specter of them in POW camps languishing, alone with a ferocious enemy across the Channel, would have induced Britain to seek peace. Hence Hitler’s failure to achieve a complete victory at Dunkirk was the turning point of the war. If Germany had won a complete victory at Dunkirk, it would have achieved the critical political and economic German objectives. Ultimately the military and economic power of Germany was destroyed on the Eastern front by the heroic Russian resistance at Moscow, Stalingrad and Kursk, the strategic bombing that largely originated from bases in England and Normandy.
BTW. Many Americans who exist today would have never been born if the Russians had not fought so heroically and had destroyed the might of the German army. Try to imagine the fight to recover Western Europe if the Russians had no decimated over 2,000,000 of Germany’s best troops.
As jmacusa said above, Hitler neglected the U-boats, making the blockade of Britain impossible plus, being unable to seize control of the English channel for lack of a navy, Germany could not have invaded England.
There would have been a stalemate. Hitler’s biggest mistake was invading an ‘ally,’ the USSR. He needed the oil and couldn’t do anything else to England. Dunkirk was not decisive.