I blame smart phone use. It's so much easier to doom scroll for your dopamine fix than go out and build experiences and socialize. I think there's going to be increasing amount of research and concern about what is an acceptable amount of smart phone use for children and young adults.
Flip side I think young people are increasingly interested in healthy lifestyles, getting drunk might not be part of that equation.
I'll speak for myself and my circle (including older millennial friends with teenage/young adult kids, such as my partner, and some younger millennials), I feel it has a lot more to do with how their social interactions come about, as well as what you pointed out regarding the interest in healthy lifestyles.
My partner's kids are 15 and 16, respectively, and they do get a lot of screentime, but mostly playing online with friends than doomscrolling on social media, and they'll still go out of their way to have friends come over, or go to friends' homes to hang out and play stuff in person every week.
Me, my partner and our small-ish group of friends, all ranging from 30 to 45, would rather a road trip or a night in with friends for board games and homemade meals than going out (and have been so since our mid to late 20s). In this group we have two couples with kids in their 20s, and sometimes they'll come hang out with us instead of going out partying, or they'll take the days their parents are hanging out at someone else's place to have friends over, and none of the kids drink (they will partake in the occasional cannabis, however)
Smartphone and social media are, indeed, an issue to be addressed, but I feel like there is something much bigger afoot here than that being the responsible for the decline in alcohol consumption and bar/club culture in general.
