First of all, it should be understood that politically, World War I without an Eastern Front would make zero sense whatsoever.
Never would've gotten to the West at all without Russia's and Germany's actions. (Mostly Russia's. :3)
Okay, so the whole August 1914 argument about troop numbers, space, supply limits, and so forth has been gone over exhaustively. The verdict of most modern historians, most notably the eminent Martin van Creveld, is that Germany in particular didn't have the space, logistical capacity, or facilities to deploy more troops than it did in the event, and probably deployed too many as it was. So as far as the initial 1914 campaign goes, it must be emphasized that
more troops on the Western Front would not have improved Germany's chances of winning the war quickly. Instead, those troops would have helped strain German supply systems to the breaking point and beyond. Whether you believe - like me and the other Zuberfags - that the whole point was breakthrough on the Trouee des Charmes, perverted by contingency and the actions of von Kluck
et al., or whether you believe - like many previous historians, especially Gerhard Ritter - that the German Army's so-called "Schlieffen" war plan was focused on a massive flank attack through Belgium, more troops than OHL originally had in the West would not have seriously improved German chances in either area.
More long-term, the problem in the West was simple. For the Germans, attriting the Allies was impossible. Wasn't going to happen. UK/France simply had more resources than Germany/Mitteleuropa did. So the problem was one of breakthrough and force density. What the Germans - and everybody else - had to do was achieve breakthrough and capitalize on it rapidly enough to keep the enemy from responding. More troops and resources - presumably freed up from the Eastern Front - is not going to change the calculus of the situation. On the First World War Western Front, the defending side always had better intelligence about any developing attack, always had superior mobility (the use of railroads, for instance, while any attacking forces had to be channeled across bombed-out no-man's land), always had the ability to easily communicate with fighting elements on the battlefield, and always was able to plug any holes and then counterattack. Adding more manpower to the attack is just going to add more grist for the mill - more targets for the machine guns - and even if they do grab a foothold in defending trenches, more men on the wrong side of no-man's land will just overload already-tenuous supply systems that much faster.
What the Germans needed on the Western Front, more than manpower, was a way to achieve operational surprise (possible with period tech, and accomplished in OTL) and tactical surprise (possible with period tech, and accomplished). So much was done by the infiltration tactics developed by Aleksey Brusilov, Oskar von Hutier, and so on and so forth. They needed to have a good way of maintaining communication with breakthrough elements, but they wouldn't have that until the radio was more portable and widespread, another two decades off. They also needed to have a way of maintaining supply connections with their forward elements, but I don't see how that's possible without tracked vehicles (in their infancy, and rarely used by the WWI
Heer) or helicopters.
And, of course, that's without getting into those nitty little problems like "what
winning the war would have actually entailed". So I would argue that more manpower and resources, freed up from the Ostfront, wouldn't have helped the Germans to reach a military decision in the West.
