First and foremost, this would do absolutely nothing to stop the negative effects of NIL & the transfer portal on G5 Schools. It doesn't matter if the G5 makes a "no transfer" rule if the P5 NCAA keeps its current policy. Players will just say "cool, good thing I'm not in your league anymore" as they sign with their new P5 NCAA team. It also won't stop those same kids from being lured away from G5s with P5 NIL deals. And it won't even prevent G5 NIL deals. The NCAA didn't want NIL to happen. They had to let it happen for antitrust reasons. And so would a G5 league.
Second, we'd lose a lot of money by doing this. The going rate for P5 money games these days is something like $1.5-2 million. We get another $1.8 million/yr from the CFP. And we'd almost certainly lose TV revenue as well. ESPN won't want to give us $2 million/yr if the product we are giving them is minor league college football instead of top 25 major college football.