Simple, they would not.
The continuation of WW2 would have gone on in other parts of the world than Europe. And Germanies basic problem would still exist - too many enemies, too much space to garrison, not enough production. And a populace that is not that interested in a never ending war against people who they are not really convinced are their enemies to begin with.
After Germany loses a few million men taking out the USSR, and is involved in a continuing occupation of Greece, France, Norway, the Low Countires, Yugoslavia, and all of the USSR, how much political will would there be to send more German troops to North Africa, or Greece, or wherever it is that the US and the UK decide to start picking away at the Third Reich?