The Maginot line was formidable, that's why Germany didn't attack it head on. The big mistake France made was its army was run by old duffers, mainly commanders from ww1, most French General's were in their 70's and 80's, utter madness. As was the belief of the old duffers that the Ardenne was impenetrable, and thus was only lightly defended by largely 2nd rate units. If the German attack had been met by by the best French and British units then it's likely the fall of France doesn't happen, certainly not in a matter of weeks.