This article has the right conclusion, but it misses nearly every significant technological change that had strengthened the defense - many of which, like smokeless powder, went back to the U.S. Civil War (which also had a lot of trenches by the end of the war.) Think railroad, telegraph, radio.

But it also misses what can change during a war. When the war began, a four-year war wasn't physically possible, because the Axis had no rich source of ammonia (beyond collecting human and horse urine) such as guano islands. But ammonia synthesis was invented during WWI, by a German chemist. Suddenly a long war was in the cards.