That is an excellent question. I will keep my answer very brief. It is possible to draw some clear conclusions. The bilingualism rate for that age group peaked in 1996, when young people reported being able to speak both official languages. Now, 15 years later, the individuals in that same cohort are between 30 and 34 years old. When you track them over time, it is very obvious that their level of bilingualism is now quite low.
It is a fact that the situation has deteriorated, but there is an explanation. Compare it to playing the piano. If you've learned how to play but don't practise afterwards, obviously you will lose the skill over time. The same goes for a language. It has to do with speaking the language and having opportunities to use it. The phenomenon is purely mathematical.