Broadly there's two scenarios:
Scenario 1: US does not join the war, but still extends unsecured credit to Entente. Result: In 1919 Germany either folds or collapses due to Blockade.
Scenario 2: US does not join war and also stops extending credits once loans cannot be secured any longer. Result: Entente folds or runs out of stuff before Blockade brings down Germany.
On scenerio 2: the Allies could just shut down any offensives on the western front in 1917 (but that might be a good thing for the Allies, i.e. less losses).
They could shut down any offensives in Palestine or Iraq and even evacuate Salonika (Greece hasn't declared war yet on the Central Powers). This should reduce the strain on shipping.
On the eastern front, without USA aid promises and less British artillery and military supplies shipped in via the Murmansk railway, perhaps the Kerensky offensive doesn't happen (but could this allow the Kerensky government to survive?, at least allowing the Russian army to be at least nominally on the side of the Allies longer. Maybe instead of shipping military supplies in on the newly completed Murmansk railway, food is shipped in instead averting a revolution.)
If Japan thought her gains threatened, could she kick in more to the Allied war effort at this point. Some divisions for the western front???
(Of course Germany has much more reserves and less conflcting obligations which might mean trouble for Italy in late 1917)
So maybe the Allies find a way to survive, the Germans don't offer any reasonable peace terms the Allies would accept, the Germans start launching offensives which eventually have to spend themselves out at some point. Even if the Germans did better than OTL 1918, can we really see Paris falling? Is France ever going to agree to the Germans having the Briery basin, Belfort or such places the Germans will surely ask for? Is Britain ever going to settle for German domination of Europe and German submarine bases in Africa?
I am a firm believer that the Allies have to do absolutely awful to lose after September 1914. That any bad situation has butterflies that mitigate things for the Allies. It might take until 1920, but Britain, all the dominions, France with its colonial empire, Japan and Italy just have to win or at least force something close to a tie, because the alternative (a German dominated Europe) just can't be allowed to happen.