I can't emphasise enough that I am not only literally a young european, living with a young european partner, with a young european social circle, but I also exist in the real world and not in reddit. I have lived here in denmark for coming up on 10 years.
Yes, I know some extremely online people who have learned better english than danish and talk about noticing a decline in their danish skills. Those few who spend all their time online, on reddit and the like. It's definitely a type, one that by my estimation skews male, autistic, and introverted. Prime hacker news demographic type thing :)
I get it, for you as a linguist its remarkable that this could happen, that people could exist that lose their native tongue or learn another to a higher proficiency of that of the country they live in. We do live in unprecedented times.
However I do not think it is universal, or a majority of young people - I also know many more people who have better knowledge in Danish than in english. It is explicitly on issues of politics, business, science, law, etc that people prefer to speak in danish with me because they do not know the words in english, including at meetup groups dedicated to those things.