Carnival as I knew it back then actually works differently than many people think - it's less about enforcing strict rules and more about each generation making their own version of it. What I find interesting: young people do take it seriously, just in their own way. They might care less about the historical accuracy of a mask, but they definitely care that the whole thing is fun and works as a community activity.
What used to be considered "real old traditions" was often already an interpretation of even older traditions. The Schnitzelbänke that are still performed haven't stayed unchanged for 500 years - they adapt. What actually matters is that people still come together at all, dress up, laugh, and celebrate something as a group. Whether someone costumes themselves strictly by the rules or just throws together some wild outfit is ultimately less relevant than the question of whether the tradition is still being lived.
That might be exactly the crux of your observation: it's less about less seriousness, but about a different kind of liveliness. If it were only about the classical forms and young people rejected those, then yes, it would die out. But as long as they're celebrating along - whether traditionally or in their own quirky way - the spirit of Carnival stays alive.