I think it's a trend to lose players their junior/senior year as they realise they don't want to play in college and just aren't interested in putting that much time and effort into the game at that point. I quit my team during summer training for regionals because I was entering college in the fall (unlike most of my rising senior teammates)and not interested in two-a-day, every day practices. I preferred to have a job and save up money to enjoy the summer. I know a lot of fellow players who made similar decisions their last year or so of high school for the same reasons: different priorities.
That said, combining ECNL, Premier, and Challenge seems a bit strenuous. A pool means not everyone is going to play all three all the time, but it's still asking a lot compared to manning one Premier/ECNL team and cutting the other players--though there's something to be said for club-player loyalty in not cutting them. I've definitely played a tournament with no subs though...it's brutal, but I enjoyed the playing time while I had it and we only did it once or twice in multiple years of playing; usually you can find one or two guest players to fill the gap. And playing 11 in Seattle isn't quite the same as 11 in Lousiana; a little more forgiving temperature/climate-wise.
At any rate, at that level and age, you're playing because you still enjoy it and want to or because you plan on playing in college. You pretty much don't hack it if you're not enjoying it; it's too much work. Unfortunately though, it comes down to growing up. Eventually you have to put down the crayons and pigtails for the pearls and heels (figuratively speaking)...and extremely competitive soccer becomes less of a priority for some.
(And thank you for the props; I owe a lot of where I am to lessons I learned from soccer).
