This was the exact scenario the western allies had prepared for and were commited to fighting.
The French expected to fight a decisive battle in central Belgium - BEF, the Belgian Army and their 1st and 7th Armies, the best and brightest of the allied forces. The Germans would be halted here and their strategical offensive capability bled white - the French expected that their own armoured forces would be ground down as well, but with the Belgian army still in play, and their much better production capacity and the front running through Belgium (and thus the coal, iron and industry in northern France safely in French hands) the French expected to be able to rebuild their armoured forces much faster than the Germans could.
The British would blockade the Germansm causing famine and a lack of important rare materials. By summer 1941, the French would be at the Rhine.