It's soccer so you need fast or quick runners to win, even if you are playing possession soccer. If you can't get to the ball before your opponent, you can't possess the ball no matter how good you are at positioning. Slower teams should play lower flights. They won't be able to keep up with fast teams and it's OK. They may or may not get faster as the years change their bodies.
I've seen plenty of teams with 1 great and fast striker but a good coach will teach them how to defend against those players. Don't let it get to the striker. Force the passes away from that striker, cut off the angle. That's learning to play defense at a young age. If the kids are unable to do that yet, can't read that team's middle, or put enough pressure on defenders to not allow those big kicks forwards, they will lose and those are the things they should continue to work on.
The build out line was created to prevent those easy goals at a young age. If your team can't play from the back reasonably with the build out line in place, well...that's something the coach should work on with the team and not force them into a higher level.
How is forcing them into a higher level going to help?
As for levels - we have so many levels in soccer right now in SOCAL - no one even knows what level is what. (created by big clubs for big payers who want to believe their kids have advanced to a higher league even though it's just renamed) We have rec level teams in club. Average is level 3 - like a C
nothing wrong with it. Flight 1 should be the best of the best. Flight 2 is fantastic but missing a few components like speed or vision. Flight 3 - Average. Flight 4 - Beginning Club Flight 5 -Rec. Most teams should be playing Flight 3 and they are but they don't even know it because Big clubs and leagues work together to rename the flights for the parents egos.
There's nothing wrong with player movement. Players change and sometimes they need a different place to train. Sometimes teams outgrow players and sometimes players outgrow teams. Sometimes teams keep weak players because it's a local team that wants to give that player a year or 2 to get physically there but still allow the kid to play on the same team as his/her friends. Sometimes great players stay at weak teams because they want to continue playing with their friends and will find other ways to improve their skills without having to train with the best players often. Choices are made by parents and kids - not by big clubs selling fake flights.