As an actuary for more than 40 years, I like to believe that I know a thing or two about risk. So I'll take off my moderator hat and add my 2 cents to this discussion.
Clearly, air travel is the safest travel there is. The only safer thing for a baby is to stay home -- and even staying home is not risk-free! So, if a family needs to go from Point A to Point B (and these points are too far apart to walk), then getting that family into an airplane is going to be safest, regardless of whether any babies are in car-seats or just wandering around the plane loose. The overwhelmingly vast majority of trips in commercial airliners are smooth, uneventful and injury-free. Those are just facts. No one can dispute them.
Are babies safer in car-seats than not in car-seats? Sure. Nobody disputes that, either. It's true in cars, and it's true in planes. Car-seats protect little babies -- and they do it well. But putting a car-seated baby on a plane today -- when nearly all flights are full -- requires buying that baby a ticket. Many families have enough trouble buying tickets for the adults, let alone buying another ticket for their baby. If requiring car-seats for babies drove even a few families from planes into cars, the overall effect on infant safety would be negative. More babies would die, in other words. The ones whose families could still afford to travel on planes would be very slightly safer, but the ones whose families switched to cars would be in much greater danger. Many more babies (and older people, too) die in cars every year than in planes, no matter how that's measured (deaths per mile, per trip, per day or whatever).
So, in my humble opinion, our society is better off putting families on planes for long-distance travel, even if their babies are not optimally protected. The huge advantage of air travel's greater overall safety overwhelms the small disadvantage of not necessarily being in a car-seat.
Bruce