It wasn't that they all thought it was impassible, they just thought it unlikely due to the restricted terrain and thus French High Command played a bet that they were going further North. (Hence the change from the original plan to put more forces into Belgium, thus denying themselves the use of the major reserve). So convinced were the French of this (or at least so determined to not consider themselves as having been wrong) that even when reports started coming in from aircraft, they didn't believe it. That said, Petain did say it was impenetrable.

However, bearing in mind that Sedan had been fought over between the Germans and French a couple of times in the preceding century, the idea of another German fight there was certainly not beyond bounds. As a result, they had a look into it in the mid/late 1930s, and made the conclusion that it would take between five to nine days to reach the Meuse, and then another good few days to actually cross it, after a week's preparation time and all that. Thus, no big hurry. This was, of course, proven completely detached from reality as the Panzers got there on day three and crossed on Day 4. Even then, some French commanders weren't so sure. In May/June 1938, General Pretelat concluded after a map exercise by 2nd Army (FR) that the Germans could make the Meuse in 60 hours, and cross it the next day. He was accused of pessimism by Gamelin. Overall, though, the French attitude was less that the Germans "couldn't", but more that they "wouldn't".

In March 1940, Deputy Taittinger of the Parliamentary Army Committee went a-visiting, and reported to the War Minister and Chief of the Army that he considered the Sedan area to be far too under-defended. The report was publicly dismissed by Army leadership, but on the other hand, the French had also started building new defenses at Sedan as well. If you look at a fortification map, you'll find there were a ridiculous amount of bunkers, though many were not completed at the time, but there was also a very surprising 'gap' in coverage, right at the Northern curve of the river where, of course, the Germans crossed.

Even at that, the local forces assigned to the area were not so complacent as to think they would not need to defend (Though granted, conducting exercises, whether they are likely or not, it just good soldiering). X Corps had conducted a number of exercises over the preceding couple of months, and indeed, just three weeks prior to the German assault had conducted a rehearsal of "actions to be taken if the Germans cross the Meuse at Sedan". As a result, when the Germans did cross, all they had to do was pull the instructions off the shelf, and issue them to the subordinate units. The units allocated for the job were certainly more than enough to kick the Germans back across the river. The problem was that for reasons of general higher command incompetence, what should have taken only a couple of hours to put into motion actually took most of a day. That was a day that the French did not have, and they lost the race to the dominant terrain at Bulson by less than an hour. And that was all she wrote.