I still think Schlieffen's idea - less of an actual Plan - was overoptimistic on the logistics side of things. (Where's Yncs on this?)
No matter how many troop the Germans could deploy there was only so much ground they could cover at a certain speed by marching, effectively limiting the plan. The wingmen were keeling over with fatigue in the later stages of the initial push. They wouldn't have been able to go further faster on their own anyway so there wouldn't be a speedier flanking of the French in any case.
Most of the real fighting was done between the Sambre and Meuse, where the French took the bulk of the million casualties in 1914, retreated, but didn't disintegrate. Possibly one might argue that more German troops there could have meant a victory, but there doesn't seem to have been an actual shortage of them in the sector so it might not have mattered at all.
Having more troops isn't a panacea. With enough of them around on both sides you get a stalemate pretty much whatever you do. Soviet military planners did a lot of arithmetic about this stuff post WWII. Generally without mechanisation things will bog down once you deploy enough people in a theatre of operations.
Besides, according the German military planning Paris should have been toast anyway, as the Germans considered it too complicated to transfer and deploy an army to the west of Paris to counter the German move. Only the French didn't know that and did it anyway by implementing the famous "système D" of the French army.
That should perhaps be considered a flaw: The Schlieffen plan somehow seems to have assumed the French to be sitting still. But as von Moltke Sr once observed iirc, no plan survives the initial contact with the enemy.
That experience might have been more radical for the French army in 1914 (nothing going right) than for the German (just enough things going wrong to land the plan in the bin), but it still holds true for both.