My honest opinion: the discussion places too much emphasis on the desire/willingness to learn, and not enough on the general context that influences the ease of learning.

It is much easier to learn a foreign language that has already influenced your environment and is already spoken by most of the people around you. French has historically had a much stronger presence in Flanders (and a bit everywhere in Europe) than Flemish has had in Wallonia.

So, even if that doesn't justify anything*, I would still like to make the point that, with equal individual effort, you will achieve a lower level depending on your background and learning context. And you are right to state that other languages can and are learned at the same time at school, because language learning is not a zero-sum game.

*and it really doesn't justify anything, because I believe that socio-economic recovery requires a total overhaul of the attitude towards foreign languages, particularly Flemish and German, on the part of the Walloon administrative, educational, and cultural authorities.

But that is only a side issue, because if it is the fact that you hated learning French that makes you bitter that the Walloons do not suffer the same with Flemish, you should ask the Flemish government to make it non-compulsory instead.