As a parent of 3 kids and an extensive baby-carrier user, I see your argument but I think you're lacking consideration of the failure modes.
Yes, almost every baby carrier would likely restrain the infant better than add-on infant belt if the adult lap belt rides up over the infant carrier, the infant's going to be crushed between the adult and the belt. Not a good outcome.
Thus you're likely going to need FAA-approved infant carriers, same way as you'd need FAA-approved car seats you can take on planes.
Yes, almost every baby carrier would likely restrain the infant better than add-on infant belt if the adult lap belt rides up over the infant carrier, the infant's going to be crushed between the adult and the belt. Not a good outcome.
Thus you're likely going to need FAA-approved infant carriers, same way as you'd need FAA-approved car seats you can take on planes.