Both leagues will operate successfully in the same territories as long as each is offering a different "product". If only one league is offered in an area, then there is no decision to be made---best players to go that league/club. But in geographic areas where both leagues have clubs, ECNL and DA offer different "flavors". So they compete for players, but not really because players who want to play high school, and want more flexibility with their time for other sports/activities/school/vacations, are choosing ECNL (we know several starters at top DA clubs who are making the move to ECNL for this reason). Those players and families with a more narrow or funneled focus on club soccer at 13/14yrs are choosing DA. If your kid has limited interests beyond soccer (not saying they shouldn't), then DA is an easy choice. But if they love soccer AND also enjoy playing the violin and volleyball, and spending time after school with Math Club, then ECNL gives them that option. Not all girls who love soccer want to narrow their career paths to being a soccer player, coach. or physical therapist from their first year of high school. A bit of hyperbole, I admit, but there is some truth to that as the girls in college now all had more flexibility with their high school time than the DA is now requiring.
Good players in both leagues will play at the same colleges. Players from both leagues will eventually get call ups to the senior USWNT. The only thing at this point ECNL players miss out on is a slim chance at making a young national team that gets together a few times a year for training/tournaments. Worth it? Depends on your kids' interests beyond soccer.