I don't believe that card collecting will reflect based on game attendance necessarily...I hardly ever go to games because I'd rather spend my money on cardboard than watch the game live
I'm a huge fantasy player and literally pay attention to every game, but just not live.
Also, baseball and football are ultra popular in this country, so I don't see them going away anytime soon or ever...at least I hope not! Card collecting doesn't exactly have an exact correlation with watching or attending the games, but in some cases if an adult doesn't introduce their child to the hobby, they may never get involved and especially for the right reasons. Card collecting has really taken a turn the last twenty years or so and it's more of a money thing than a collecting for the love thing...that is one big reason I love this website along with the members of it...although a bunch collect expensive cardboard here, we also love and appreciate what we collect.
I can't speak for everyone, but I've always been a collector although I've taken a few years off before, but the chances someone will collect when they become an adult probably aren't greater if they don't collect at some point when they are a kid...not in every instance, but it seems very logical that it happens this way a great deal.