I think you are conflating two, very different things :
how a language acquires a place of social prestige and dominance, and how a language replaces another in the population.
Now of course, a language which is associated with high-status in a society, is in a good position to replace a low-status language, but, as you point out, it takes a lot of time for one language to completely replace another.
On the other hand, a language can win a higher status quite quickly in a period of social turmoil, such as the french revolution, during which the french society achieved an unprecedented amount of centralization, which brought unprecedented prestige to the already dominant Parisian standard.
And it was at this time, too, that Wallonia came under the direct control of France. But it had been in the sphere of influence of the Kingdom of France for far longer.
Furthermore, once Belgium was separated from France in 1815, Europe was completely different from what it had been in 1795.
Feodalism was dying slowly and the ideology of the nation-state was on the rise.
Because of religious differences, Belgium soon seceded from the Netherlands and became independent in 1830, and in the new state, French speakers were socially dominant and remained so for a long time.
Now the Walloons actually did speak Walloon and not French in their everyday life, pretty much like everyone in every region of France actually spoke in their own dialect, but French was the prestigious written language of the elites, and the elites in Bruxelles and Wallonia gravitated towards it also.
And so French replaced Walloon not over the course of the 19 years, but over a whole century, as mandatory instruction, increased population movements (especially from the country to the cities), and mass media progressively erased French dialects.
And it's no coincidence that those that were closest to Parisian French suffered the most, because they were the ones that had the least chance to be recognized as different languages instead of just a low-register variety of the mainstream French.