Only problem is he wouldn't have left North America to do its own thing. He would have taken German advanced weapons (long range bombers, advanced missiles, turbo-jet powered aircraft, nuclear bombs, etc) on further to completely leap frog us technologically and then, from a position of strength and complete consolidation of the European continent, would have been able to launch an initiative to bring North America under his absolute dominion. Yeah, if we had remained isolationist he might have given lip service to pledges to leave us along while he consolidated Europe, but in the end he would have reneged on that like he did to all the others he invaded. And by remaining neutral our own military capabilities would have remained stunted and embedded in 1930s tech, and would have been woefully outmatched by radically superior German military tech. So it really would have been The Man In The High Castle scenario here in North America.