Without implying any callous attitude on the part of the Great General Staff, any success, let alone conquests, for the Mexicans --- even with heavy casualties---, would be less important than the merest diversion to keep the US preoccupied for a few months.
The Idiot Press would have worked into a froth about Mexicans, American Statesmen would have planned gigantic Walls to no purpose, and Pershing would have matched up and down Mexico.
Americans have always been a mite touchy about their beloved little brother down past the Rio Grande, and their tiny minds would have exploded at a full invasion.
Anyone remember how in 1960s to 80s American films dealing with Mexican revolutions and the like, the hosts of unbelievably incompetent uniformed Mexicano mooks had a few sinister German advisers attached to each squad ? Generally in white undress uniform, with nose-rim eyeglasses.