Even assuming that Germany attacks France through Belgium as per OTL, the delay in French mobilisation could easily work to the detriment of Germany.
With a delayed mobilisation, France doesn't hurl its best forces at the frontiers, but these are gathered more slowly in the reserve area. Germany is still going to get delayed passing through Belgium. The German advance was about as fast as it realistically could be, and will come up to the Marne in pretty much the state it was OTL. Only in this version, it comes up against the best of the French forces which have not been weakened by ridiculous marches (the Schlieffen plan was clearly designed by someone who didn't have a clue about the effects of lots of travel on the fighting capability of troops).
With a delayed French mobilisation, deployment of the BEF will also be deployed, and the timing is such that it could easily be that just as the Germans are being held at the Marne, the BEF is deploying near the Channel ports, in an ideal position to win the Race to the Sea. The BEF gets astride the supply lines, German armies 1 to 5 suddenly find themselves without supplies and facing troops in the front, on their flank, and in their rear. Goodbye German armies 1 to 5, and the war is over by Christmas with Germany suffering a humiliating defeat.