Except that the introduction of the convoy system put paid to the U-boat threat,
Make that "contained" and I'd agree, but the average monthly rate of sinkings didn't go below 300,000 tons until the second quarter of 1918 - long after the introduction of convoys.
Granted, that was a big improvement over the average of around 600,000 in the first ten months of 1917, but even 300,000 a month is a serious rate of loss, and represents a lot of dead seamen.
Further, if we are assuming a German victory in the continental war, that gives them the use of French ports for both their u-boats and surface warships, so expect this toll to start climbing again. It also means a terrific blow to the morale of the seamen, who are now getting drowned for no greater purpose than to avoid the payment of an indemnity and the loss of a colony or two. Will they put up with that indefinitely?
nor was there any chance of Germany winning the war in the west in terms of taking the American or British armies captive and the vision of the US even being able to pay the entire war costs of the Central Powers, let alone willing, doesn't work.
Well, that was President Wilson's fear, and, lacking a ouija board, I have no means of taking it up with him. However, I do know that as late as June 1918 a smart guy like Lord Milner was worrying about how we'd extricate the British and US armies from France in the event of a German victory there.
That said, I would agree that the AEF at least would be unlikely to be captured in toto. Unlike the BEF, it had virtually unlimited space in which to retreat, and in WW1 a retreating army could nearly always retreat faster than an advancing one could advance. As Confucius (on highly doubtful authority) say, "lady with skirts up can always run faster than gentleman with trousers down".
Wilson, very much an armchair warrior, may well have been getting overly jittery. But war is an uncertain business, and even if most of the AEF gets away, a significant number of troops might be unlucky, so that Germany still gets thousands of American captives.