The issue wasn't so much the emphasis on the Maginot Line; it was diplomatic failure of appeasement. tl;dr French security strategy relied heavily on a strong a

The issue wasn't so much the emphasis on the Maginot Line; it was diplomatic failure of appeasement. tl;dr French security strategy relied heavily on a strong alliance with Belgium, and they threw that away in 1936-8 Since the First World War, when Belgium's territory had been used by Germany as a route into France, Belgium had been in an alliance with France. This meant that while they built up fortifications on the German border, they expected to use the Belgian fortifications (places like Fort Eben-Emael) on its border with Germany as a northern extension of Maginot. When France and Britain failed to oppose the remilitarisation of the Rheinland in 1936, however, Belgium lost confidence that France would actually honor its security guarantees and declared neutrality again. They extended the Maginot fortifications along the Franco-Belgian border, but not didn't have the time or money to build it up to the standard of the rest of the line. Meanwhile, in 1940, instead of being manned by a fully-mobilized wartime army and probably elements of the French and British armies, the Belgian fortifications were held by skeleton crews of an only-moderately-alert neutral army.