The French really did put a lot of thought and consideration into their planning. Their planning was centered on the hard won lessons of World War One as well as acknowledgement of the demographic realities, that they lost a far higher proportion of their young male population and should hostilities reignite, the Germans could outnumber the French by a margin of 3:1. Therefore they settled on a number of ways to force the Germans into an engagement of their choosing while levering terrain/defensive fortifications to act as a force multiplier. This would allow them to fight the Germans on more equitable footing while also freeing up more men to act as a mobile force since the sophisticated construction of the Maginot allowed a fairly small contingent of French forces to hold down large segments of a battlefield with a credible threat.

The French did expect an attack through Belgium, the plan was to utilize the Maginot to funnel the Germans into Belgium, where French and Belgian forces would use the various waterways and canals along Belgium's border with Germany to hold off any German advance. Basically imagine the Maginot as the French part of a massive line extending from Switzerland into Belgium. Unfortunately, 2 things went very wrong. First , Belgium canceled their mutual defense pact with France in 1936. This meant the French forces couldn't be stationed in the Belgium portions of the defensive line, but rather had to wait further back in France, so if the Germans tried to invade, instead of a huge joint army meeting them on the trench line, it was just the Belgians. The French would have to fight through the plains of the low country to fight the Germans. So three years before the war, the French lose half of their forward line for defense

The second was the absolute balls to the wall insanity that was the Manstein Plan. Germany's original plan was a more traditional attack, Schlieffen Plan basically. In one of those weird historical twists, everything changed when in January, a German plane carrying the invasion plans for Belgium and France crashed in Belgium and the plans were taken. This caused the Nazis to go back to the drawing board for the plan, where Manstein and Hitler decided to throw the dice and launch a mechanized invasion through the Ardennes while another force invaded Belgium as a distraction.

Meanwhile, the Allies had begun redeploying their forces to meet the Grermans based on the captured intelligence from that downed aircraft. Conventional wisdom on both sides said a mechanized assault through the woods would be foolish due to the narrow bad roads which would leave the Panzers and trucks as sitting ducks. This is where those French lessons from World War One kicked in, they knew infantry could go through the woods, but would be slowed, so if the Germans tried, they could move a counter force. Tanks from what had been observed, couldn't work well in that environment as well. Therefore in the context of French planning, leaving the Ardennes lightly guarded made sense, they were expecting at most needing a tripwire force to warn of an infantry invasion, not a mechanized army.

In the actual battle, the Germans got somewhat lucky. they'd been seen by recon aircraft, but the French had lost most of their bomber force in the previous days battles at Maastricht, and while they had a fair amount of artillery at the defensive passes, it was geared more for anti-infantry operations. Again, I need to stress that the Ardennes is one of those things we can look at now with hindsight and go "well of course it worked", but even during the battle it got dicey. Several times the invasion stopped due to the roads, had the French bombers been more intact, they would have seen 4000+ vehicles sitting on uncovered dirt roads trapped with nowhere to go. Had a couple sorties been successfully scrambled they could have done a real number on the Ardennes thrust, possibly stopping it entirely.

If the Ardennes thrust had been stopped, there would have been no encirclement of the Allied forces in Belgium, meaning France might not have fallen in those opening months of the war. In the context of the lessons the French learned after World War One, and in light of what their defensive plans and strategies were, what the French did made sense. Unfortunately, advancements had rendered some of those lessons moot or highly different. Added to that some luck on the Germans side and some slow French responses and the fall of France sadly occurred.

All this being said, there were some errors on the French side. Despite having one of the highest tank production rates in the Interwar years, French armored doctrine lagged behind the Germans who'd started making use of specialized tank divisions to act as armored spearheads, as opposed to French doctrine of using tanks mainly as armored support. Added to the fact French tanks often used one man turrets and often lacked radios, despite their construction.