I don't think many people understand that this affects the non-trapped players as well.
Fortunately, this is a couple years away for my oldest, but this is how I understand it will work (if there are others who have suffered through this already, please correct any mistakes that I've made).
During senior year, those trapped players come down and join the younger team. Those new players will increase the roster size, potentially up to 18+. What if the team was already at 18? Then the team is either making big cuts or it's going to 22+. How does a roster of 22+ work? Many of the players don't suit up for game day. If the team is lucky, their coach will tell them before everyone has to drive to that SD game. If they have a jerk of a coach, they might just get to enjoy a nice weekend vacation to AZ to watch their teammates play (but don't worry, the Papago Park hike is lovely, just go early because it heats up most days).
Trapped players get this awesome experience twice. Not only do they get to enjoy this setup for senior year, they also get to live it their junior year as well.
Fyi, some boy ECNL teams have 30 players.