Before the war started, France and Belgium were negotiating the building of an extension of the Maginot line into the Belgium border with Germany.
One thing that France briefly considered, was to just build the Maginot line all the way to the sea, across the Belgium/French line. But this was immediately rejected as it would antagonize French Allies. Not just Belgium.
Like I said, at Sedan, there was the French Second Army. Which had been reinforced with reserve troops. And had Maginot Fortifications, though not fully completed.
If Huntzinger had stayed at Sedan and accepted the offer from the First Army to receive air support, it would have probably held long enough for the French mobilization.
To the North of Sedan there was the French First Army. And in Belgium, there were several of the best allied armies, from France, Britain and Belgium, because they thought the Germans would attack from there, since that was in "Fall Gelb".
The Germans broke through, not because of the Belgians, not even because of the French soldiery. It broke because of incompetent French leadership.