Not sure how you make the leap from "players in international frame pre major tournament play in Auckland for their own development" (which implies limited timeframe for a couple of players) to "send all our best players elsewhere". If you were national team coach would you really want players in the mix for an Olympic tournament wasting their time with double digit scorelines?
And if you can face a harsh truth, those truly committed to progressing their game, and are brave enough to test themselves in a bigger pond, recognise they need to leave the Mainland at some point anyway.
As you say, the standard is already there for the the good teenage players: already active for 40 weeks a year - from pre-season to ACFC and other tournaments, winter season, reps, U19, Pride or FTC and Nat Age Group tourney.
Agree with you that we need the increase the player base. To have your premier competition made up of 10% of your total player base is the problem. It's actually far worse - if you looked at number of WPL players as a % of say 15-30 year old female players.
I hope some discussion happens between not only Mainland and local clubs (view seems to be, "be patient, wait for Selwyn and Wiamak juniors to come through"); but also input from National team selectors. The current WNTU17 coach - a Mainlander himself - picked just one Canterbury player to go to World Cup Qualifying tournament. Ask him and the other national team selectors why they are only picking 1-2 south islanders for national teams at present; just to get a sense of perspective.