Cutting it off could knock China out of the war.
Not in time for the Japanese to be able to redirect any resources to shipbuilding or aircraft production which was where the Americans nailed them in the Pacific. In the most generous circumstances if China's out early, the Japanese can maybe get a few more ground Divisions and aircraft stuffed into New Guinea and the PI. The US will find a way to work around Japanese strongpoints.
Of course, even in terms of ground forces, the biggest problem with the Japanese getting a lot of troops out of China is that they'll likely insist on occupying a lot of Chinese territory.
The easiest alternative front that Japan could move some troops to if China drops out would be Manchuria. Here again, they could add some defensive strength, but not provide a viable offensive option.
China suffered. China didn't surrender like France. Chinese people were brave and helped out bailed out American fliers. Chinese forces were an important part of the successful liberation of Burma. Chinese forces repelled a Chinese offensive in 1945 and began to retake some ground. They did all this, but China did not crush, defeat or even drive out the Japanese.
https://chinachange.org/2015/08/31/...y-in-the-fight-against-japan-in-world-war-ii/