I guess I'm not understanding why being a trapped player is such an impediment to a kids development. It seems your major issue is that kids that are in 8th grade that play with 9 graders are at a big disadvantage. Kids play year round, I see having a few months off from game play as a benefit and is a great time to unwind and cross train.
Plus on the boys side, MLSN doesn't allow HS play, and any level of play below that is effectively recreational soccer (even some of MLSN is glorified rec).
As a matter of perspective, my son is a late September birthday and was a late boomer to boot. He was on the wrong end of the BY change, but it didn't have a negative impact on his soccer development. In fact, I'd argue he developed better skills because he was always going against bigger kids, whereas other kids would just rely on their size. He was identified for the MLSN late bloomer program but never participated. I think bio-banding is mostly BS. The problem with US youth soccer is player identification not RAE.
The perception of trapped players and RAE is far worse than the reality. In my experience their impact is negligible, and temporary at worst.