This.The weapons were manufactured in France, but the raw materials needed to make them like iron and steel sure as hell weren’t from France. They were from the US. And if the US doesn’t enter the war, the Entente goes bankrupt and the French can’t buy anymore steel.
The fact that during the Spring Offensives, there were those in France saying that Joseph Callaiux should be pardoned and made Prime Minister, even with the Americans in the war, shows just how precarious France’s position was even a year after American entry.
Add to that that the main problem with Kaiserschlacht was overly hasty prep and a failure to clearly ID strategic and operational objectives (which led to muddled logistics at the worst possible time), and quite simply, a lack of American entry means less pressure on the CP at the weakest possible moment for the Entente, which means that France will all but certainly break at the alt-Spring Offensive and the CP will win.