Actually the Maginot Line is a textbook failure to follow a basic principal that goes back so far that it ca=n be seen as instinctive. The French utterly failed to secure their northern flank on a solide defensive position, largely, but not exclusively, to avoid insulting the Belgians. The Line should have ended at the sea, either including Belgium behind it or along the French-Belgian border leaving Belgium to its fate. Instead the French did neither. By doing so they left their Northern flank hanging entirely in the air and provides a 200 mile wide entry for the Heer to exploit. There were political reasons for the decision, but from a military perspective the decision was far beyond idiotic, Of course the politicians were making the decisions based purely on political considerations.
This fails to account for a very broad range of reasons why these fortifications were not built, and why they would not have been a good idea to have been built:
1)Construction and planning of the Maginot Line took place in a period in which the Belgians were close allies of the French, so inherently Belgian fortifications and cooperation could be assured. As it turned out, they could not, but in the planning in the late 1920s there was no reason to assume otherwise.
2)The Maginot Line had a wide variety of objectives. Being an impenetrable wall was never one of them. These objectives included providing a fortification line which would protect key resources along the frontier, economize troops, but most importantly provide time for troop mobilization.
3)Economizing troops is perhaps the most important, since the Maginot Line was designed to free up mobile forces for use elsewhere, ie. in the North.
4)There are important construction problems concerning the Northern frontier, as much of it has difficulties with a very high water table. I would recommend purchasing some books on the Maginot Line which discuss its construction principles to see just how deep many of the structures were built: in a high water table region this obviously poses a problem.
5)Doubtless it is possible that this could be dealt with, at great cost: cost that would have to come from somewhere else. While the actual sums spent on the Maginot Line as a part of total French expenditures throughout the 1930s was much more limited than is generally thought, proposing a doubling of the line, with much more expensive construction, is inevitably going to eat away at funding for procurement and mobile forces - which was the exact opposite of its point, in conserving forces in one region to free up those for another, as mentioned previously.
In fact, to quote
French Foreign and Defense Policy 1918-1940: The Decline and Fall of a Great Power, these exact concerns were clearly stated by the French themselves:
General Emile Ricard, later explained, in regard to the Franco-
Belgian border: obtaining ‘effective protection [through fixed defences] for this
part of the front would have demanded an effort truly disproportionate to the
means we possessed’. Fortifications facing Belgium ‘could only have been
conceived as a device to enhance…the defensive combat power of the main battle
corps deployed there’.73 Maginot-grade defences from Montmédy to the Channel
would, according to an estimate from Gamelin, have cost between ten billion and
fifteen billion francs. This was a sum of a magnitude that France did not possess,
over and above the investments required for renovation of the field army,
expansion of the air force and modernisation of the navy.74 By 1937, judges
Anthony Adamthwaite, ‘given the state of the French economy, it was clearly too
expensive to extend the Maginot Line to the Channel ports’.75
6)Even if it was the case that the French military had pulled from the aether this tremendous amount of funding, the Northern border includes a variety of population, industrial, and economic centers, which are not simply close to the border and hence requiring protection by fortifications like in Alsace-Moselle, but rather
on it. What do you do for the city of Lille for example, which is essentially on the border? If you build fortification lines, these places are not protected, but simply become the front line and are intensely vulnerable to destruction. Strasbourg faced a similar fate and had to be evacuated, but evacuating all of these areas would simply serve to harm the long term French war making capability.
7)To some extent it was
desired that just such a hole be left in the north: the French military in the Interwar, exceptionally fond of intense planning and logic, effectively managed to dramatically reduce the level of front over which it had to plan to fight, making it likely that the Germans could only come on one approach vector - one which would preferably not be on French soil and hence not fought there. As it turned out, the Germans could come through an unexpected approach even with this, but it nevertheless did remove much uncertainty.
While the French defensive strategy ultimately failed, and failed quite disastrously, I feel that you are critiquing the Maginot Line to an excessive degree, and on the wrong points. Building fortifications to the sea carried a huge length of drawbacks and the French were right to instead choose to press for forward defense and use of mobile forces: that these failed is not necessarily connected to the Maginot Line (although the Maginot Line being extended a few dozen more kilometers to cover the Ardennes would certainly not have been a bad idea....)
The French also had budget concerns, as the Maginot Line ate up a huge part of their defense money and arguably was part of the reason the French army-air force was so unprepared for WW2. The French did bet on the Belgian forts to anchor their position when they moved in their army to the Dyle, but the Germans managed to take their centerpiece position, Eben Emael, with a quickly and shockingly successful commando operation at very low cost. So beyond their military mistakes the French (and Belgians) suffered pretty badly from having invested so much in a fortification line that left too little for the military to keep up with their opponents.
Maginot Line expenses were allocated and spent in the period of 1930-1936. Even if there had been a political will in the Maginot Line votes around 1930 (which there was not - defensive spending was popular and securing the borders even more so, offensive nature spending was politically impossible, and
even if was voted for then it would probably have been, unlike the Maginot Line, been cut), then assuming that funding was directed to the French Air Force instead the aircraft it would have built would have been painfully obsolete. Of course, there would probably have been positive secondary effects including things like airbase construction, airbase defense, pilot training, communication systems, building up productive capacity, etc., but spending money on aircraft construction in 1930, 1931, 1932, etc. only is going to have a very limited and secondary impact on forces available in 1940 or even in 1938. More likely the funding would have been swallowed up in the inefficient building apparatus, to build planes which would have already been mediocre in the early 1930s, and utterly obsolete several years later... the Maginot Line fortifications at least, preserved their utility in 1940, and even half a decade after that, as can be seen by the shambles of the American attack on Metz.
The same can be said concerning the army but to an even greater degree: the French Army received 14 billion francs for a 1936 plan alone to modernize, which is nearly 3 times Maginot spending. But if the Maginot Line was never built the French would have had to station far more troops along the Franco-German border. They did station far too many historically, but this would just amplify it. The number of mobile forces available to fight in Belgium would have been reduced, not increased.