I'm going to stir the pot a little, I'm afraid...
Plane crashes, "survivable" or not, are much more rare than car crashes. Air plane travel overall is hugely more safe for everyone than driving in a car.
I have never been able to dig up any really good statistics, but I suspect that flying somewhere without a car seat is safer than driving to the same location with a car seat. And this is the rational for why children under 2 are not required to have a ticket. All things considered, the people who make these decisions have decided that it would actually increase the risk to the child to make the trip in a car (with the car seat and all) than it would to have the child fly on a parent's lap. By requiring parents to buy plane tickets for their infants and young toddlers, they feel it would increase highway fatalities and injuries because more families would choose to drive.
Unlike flying, driving is one of the most dangerous things we do with our children. Possibly THE most dangerous thing we normally do with our children.
When my family went to Hawaii this year, we did not buy a ticket for the baby. We did, however, bring her car seat on the plane with us. Only on one of the inter-island hopping flights was the plane too full to use it. And on another one, we assumed it would be too full so we gate-checked it only to find out the plane was half empty. But she used her seat on both the mainland flights and on 3 out of 5 of our island hopping flights. All without buying her a ticket.
Of course, driving to Hawaii isn't an option, but if I were going to, say, Florida, and I had to choose between flying with the risk of having the baby on my lap if the flight is full, or driving with the car seat in the car, I feel a lot more safer flying.
We don't get all bent out of shape when children ride busses or trains or light rail without so much as a seat belt. Why are airplanes any different?
Would the child be safer with a car seat on the plane than without? OF COURSE. I'm not arguing that point. I also wouldn't check a car seat as luggage. And if I could afford to make the trip while buying my child a seat on the plane to be SURE the restraint could be used, I would. (Although, even then, I've heard enough stories of people not being allowed to use their car seats even though they bought a ticket I'm not sure that's even guaranteed). But given the rarity of plane crashes, I do feel that experiencing family vacations is important enough of an experience and bonding time for my family that I do not believe it's better to just stay home than risk having your baby ride on your lap on an airplane.
And yes, I know occasionally turbulence can get really bad, but if you're wearing your seat belt (as you should be) and holding on to your tot it's really not the same thing as trying to hold your baby in a collision. Would it be hard, tiring, and have some risk? Yes. But life has risk in it. We do our best to limit the risk (hence car seats in cars, and when possible, in air planes) but sometimes I think the risk of living in a bubble (and not exposing your children to important experiences) is higher than the risk of something bad happening. If I wanted to eliminate all risk of injury during transportation, I'd never take my child in a car with or without a car seat. And I still believe (until I'm proven otherwise) that air travel without a car seat is less risky than car travel with one. So if I'm willing to take the baby with me to the grocery store, I ought to be willing to take the baby with me on vacation. Even if I can't afford to get her a separate ticket.