No technical reason why the .50 anti-tank rifles could not have been around in 1914 instead of 1917/1918. For the tanks of WWI this works pretty well. Really can't have tanks much earlier than OTL due to the inadequate engines, however with a bit earlier engine development might see armored cars in Russo-Japanese War, maybe Balkan Wars and this would prompt earlier production of .50 cal anti-tank rifles. This will not make tanks "obsolete" however.