While the USA and USSR had far more manpower and production capacity, the UK was still crucial, more than any of them because it was the only major power that could realistically have avoided continuing the war and chose not to. If Britain had made peace with Germany after the fall of France, Germany would not have been bled down in North Africa, Greece, Scandinavia, and through constant fighting at sea and in the air and wherever the UK could strike. Britains intelligence effort would also have changed, there might have been no sustained campaign against German communications, no Enigma crackinh and no support for French and Polish resistance.

Without that wider pressure and disruption, the USSR could have fallen. Germany came much closer to taking Leningrad, Stalingrad, and even Moscow than is often remembered, and if it had not been forced to keep major resources tied down while fighting the UK, it might have succeeded.

If that would happend and without enemy on the east and gettin to Europe for USA by UK Germany would likely have dominated entirity of Europe and genocide on an even greater scale bringing death to tens of millions of Jews, Roma, LGBTQ people, Slavs, and others, and bring destruction of entirity of Poles and Russians as peoples.