I can't find it now, but the FAA did do studies on how many passengers said they would drive instead of fly if they had to buy a seat for infants, and if even 25% of them did drive instead of fly, that would increase deaths tremendously.
For an ideal solution, car seats and seats would be required, but they would be half price. Airlines would also offer FAA approved car seats for use on the plane. Fund it through car seat manufacturer donations or other charitable donations. You book the seat for an under 5 let's say, and you can mark if you'd like a RF car seat or a FF car seat. That eliminates the cost and the lugging car seats issues.
Of course, for an ideal situation, airlines also wouldn't increase the danger for everyone on the airplane by starting their inane and dangerous baggage fees. That's led to millions of people lugging heavy bags onto the planes and putting them over the heads of innocent passengers in order to save far less money than an extra seat.
The next page in that book linked above mentions how those bags kill and injure people by blocking exits and falling on people. Like the flight attendant there said "Luggage compartments are for bags. The cabin is for people." I've certainly seen hundreds upon hundreds of bags that should not be above my or my children's heads in baggage compartments that can fly open even without turbulence.
I watched an idiot on the plane this weekend pull his overstuffed bag out of the overhead so he could get painkiller. Meanwhile, he smacked the sleeping woman sitting under his bag in the head.