The reality is that lap infants are permitted because it's sensible on a basis going beyond mere economics for families. Given all the risks that there are on the ground for U2 children, there are way bigger risks in the world for the average U2 child on the ground than there is in the air for the average U2 child. It's a good thing that the US airlines and their backers haven't yet been able to pull off a general ban on lap-children on planes.

Of the millions of miles I've flown with lap children on board, I've seen more U2 children hurt on the way to/from/during diaper changes/bathroom use and during boarding/disembarking than while the children's caregivers are seated on the plane along with the U2 children. Should we ban bathroom use by U2 children as an idealist and mandate that diaper changes and U2 bladder/bowel relief be done at the passenger seats since that may reduce injuries? I would be opposed to such a ban too.