To get the Line extending from Switzerland to the sea we're implicitly assuming pod's that (ai) include a preference of investing in the ML over all/most other civil/military projects for a period of many years or (aii) a lesser impact from the Depression and issues around the devaluation of the franc and (b) a relationship with Belgium such that (a) the immediate pre-war Belgian policy of 'no allies' was established longer or (b) the French weren't concerned of the foreign policy consequences of putting the Belgians between the ML rock and the German hard place
I do not see France falling to a German landward attack. OTL France fell because the ML was outflanked, the main German thrust occurred at hinge of the Anglo-French advance into Belgium and the French strategic reserve was despatched into Holland. As I'm attempting to show, there is some credible doubt about the overwhelming success of the German attack being replicated under the assumption of the abandonment of the Holland advance (the Breda Variant to the Dyle/Escaut Plans).
The Germans chose not to attack the ML in OTL for a good reason: it was an excellent defensive position. In terms of economy of force, piercing the line would have been a poorly remunerative operation from the German point of view and there would have been serious doubts in OKH about whether or not the German army could bear such an ordeal. There may have been a challenge-and-response result where the Germans sought a technological advance, but such an advance would, I suggest, bear little resemblance to what we see as classic blitzkrieg dispositions.
Of course, as I've noted above, the France of a complete ML would be very different from OTL and, hence, the Europe of 1940 might be very different as well.