As a millennial who just read this thread, as I went through I found myself liking most posts concerning costs of the sport, costs of overall vacations, and the different resources available to millennials at this point in their life course compared to previous generations: relatively little wage growth, an increase in income inequality, jobs that expect longer hours and relatively little vacation time, and much larger student loan debts on average. (Looking at many peers, I feel "lucky" to only be paying $300 in loan debt a month. But that is a pretty crazy amount when I compare it to other monthly bills -- my $0 car payment, my $900 spent on total housing expenses, etc.)
Now I am obviously not a representative millennial -- I grew up skiing and organize a large portion of my time and resources around it. (Seriously, its crazy to think about: for my ~50 days on the hill a year, I'll spend ~7% of my net income, and 6 of my 12 PTO days -- that's an insane allocation.) I still talk to a lot of people I used to ski with when I was a youngin' who loved it then. Only 2 of them still ski -- one like me who has chosen to make it a major focus of his life, the other his sister. To the other folks out there I encourage to come join me -- skiing is a shitty value proposition. Millennials were trained to admire adventure and thrift -- and compared to seeing another country skiing is hardly an adventure. And when I can go to France for 10 days, drink world class wine, stay in beautiful places, and eat good food for less than what a comparable week would cost saying in many US ski destinations....its no wonder we don't come to the hill.
Part of the problem to me is also that millennials see the mass crowds at many places, and decide its not worth it. (Especially with a sport that many equate with some peacefulness.) So more boomers skiing today (i.e. more and more people on the hill) means less millennials today and in the future. Boomers are more likely to spend $20 to avoid taking a shuttle from the parking lot, more likely to spend $250 on a hotel for a night to avoid driving to-from the next day, buy a "secret pass" to skip lift lines -- all these make the crowded situations more manageable. That's great for the current bottom line, but it doesn't train a bunch of skiers to get hooked and make it part of their yearly practice.
I think to understand this problem with more accuracy we need much more granular data: where are skier visits decreasing generation over generation? Regions, types of hills? Is it day trippers decreasing or family vacations?