More reasons it failed
The actual war plans of the German Army began to become more and more corrupted by the Plan, existing and being modified to serve the Plan rather than reflect reality. By the time it was actually tested in conflict (the beginning of World War I by von Moltke the Younger & co.) Schlieffen had already disowned it as being the wrong tool for the job.

One reason the plan relied so heavily on speed is that the German war plans called for a linear two-front war; that is, they would quickly smash France and then swing their forces east to deal with Russia. This is an example of your version of reality being modified to fit the plan rather than vice versa. In practice, the extremely tight timetable of the plan and the enormous complexity of its logistics caused the operation to collapse. Surprise, which the Germans had achieved in many areas of the front, was lost as supply and reinforcement channels 'broke.' The delay gave the defenders time to erect trench defenses.

Although neither side had really planned on trench warfare, as both sides were married to the offensive, the French were pushed to it by necessity. Once started, the trenching didn't stop, and soon the entire war was being fought (on land in Europe at least) along a narrow strip of the continent along which the trench 'cities' were erected (or rather, dug).