Umm - you realize that they (and everybody else) always has the ability to play up. Call it a 15 month cohort, a 30 month cohort, or a 22-year cohort. If you are under <this> age, you can play in <this bracket> and any bracket above it that will have you. This has always been the case. This isn't a new (or coherent) idea.
What you're basically proposing is that the summer kids can play down in age (they would be too old otherwise, but are in the lower grade because they started late).
Moving the actual cutoff to 7/1 - means that the oldest kids are now 7/1 (duh), and the youngest kids are now born 6/30 (and are playing in the next older age bracket). And the ones 12 months younger, born 7/1-8/31 the following year, now have the unenviable position of either playing down a year so they can compete, or now being up to 14-15 months younger than the kids now allowed in the same bracket.
Nothing here is gained for free. The lacrosse model makes it a 15-month calendar, essentially from Jun 1 to Aug 31 the following year, with a hard stop - requiring grade proof for a summer kid who chooses to play down.