In very general terms, once UK, USA, and USSR are in the war on the same side, any Nazi victory scenario is pretty much dependent upon
If the Allies are anything other than wilfully stupid to a ludicrous degree, the outcome of the war is pretty much settled and all that remains are the details on how and when that outcome is achieved.
It is true that Nazi victory would probably require either keeping the US or the USSR out, or the UK withdrawing from the war before either of those entered.
From what I understand, it seems that the problem is that it is somehow assumed that all the leaders of 1940 believed that Germany wanted to conquer the world but at the same time was so pathetically weak that it was just a matter of "showing toughness" and holding out until begin to collapse, no matter how long it takes and no matter how much has to be sacrificed in the name of inevitable victory.
Which is not so clear to me that it was believed at the time. In 1940 there was generally not the degree of fetishization and obsession with the economy that exists today. Even if it had been shaken by the First World War, the idea that determined, well-led and morally high soldiers could defeat materially and numerically superior enemies still existed to some extent. (This was the war that decidedly buried this assumption.)
Nothing I have seen in the descriptions of history makes me see Churchill, Stalin and Roosevelt crunching numbers on a pad to conclude that "our victory is inevitable, we have economics on our side." Or that they would decide to absolutely ignore every factor other than the economic one, which is what I have seen here and in other threads in various answers: people "operating" under the assumption that the allied plans are based exclusively on economic assumptions and cost/benefit relationships.
Ironically, I think the Allies are credited with what I have seen called here "the fascist view of the enemy": the enemy is both immensely strong, an existential threat that must be contained at any cost, and pathetically weak, enough just show a little firmness and they will start to fall.