Without unrestricted submarine warfare Germany has no weapon to use against the UK. The British continue to control the seas, the Germans starve.
But you're right, with the submarine warfare you get American intervention. US would not be able to stand still if their ships are attacked.
But if there is a ultra isolationist US President then the war could end sooner rather then later.
US money and industry kept the Entente fighting.
What happens if the US doesn't supply guns, munitions and sundry other items that they consider to be contraband.
Remember the US sold battleship guns and submarines to the entente in contravention of the hague.
A truly neutral US would cause immense problems for the Entente as the US might just force GB to allow Neutral to Neutral trade.
And some of those neutrals Sweden, Holland and Denmark are next door to Germany and in the first two cases oh so friendly with Germany and wouldn't mind on selling to make enormous profit.
In the case of Holland, Queen Wilhelmina was very anti British because of the British action in the Boer wars to the extent that she supplied cheese to the German troop to stave off hunger.
Another biggie is that the US might force GB to allow Direct trade between the US and the CP.
Wilson lost a lot of support and had to strong arm GB to buy some of the cotton that was contracted to be sold to Germany
Another problem with the US and GB is if GB causes problems like seizing US ships it might get the US into the war on the CP side.
I'm quite sure the GB would trade Canada and its Atlantic territories to try to defeat Germany.
The end result would be a German win or draw with FRance and GB losing big time.