The AAP is a medical organization..I see no issue with someone providing test data to them as it makes sense one relies on experts to provide them with safety information and data.
There are many carseats that last a really really long time. Its entirely unrealistic IMO to think a seat should last from infancy until they are tall enough to be in a seat belt. There is more than 1 seat on the market that will take them from rear facing infancy through to a booster seat and that is extremely reasonable.
As for the age of a child when they move to a seat belt..I think there are a few factors there..the basic fact is that if you are too small height wise or not heavy enough weight wise the seat belt can not function properly and as such seriously compromises your safety. So it makes complete sense that a child should reach a certain height/weight requirement before using them otherwise they are not safe because the seat belt is not functioning properly and they are at greater risk. I understand that some adults barely reach those heights..my own Great Grandma was probably barely 4'10" however those are the exceptions not the rule and the rule applies to the average/majority.
Also as children age some of the bones and ligaments in their body are not fully developed yet..what the neck and spine of a 1 year old looks like vs a 6 year old is vastly different..meaning the body as they get older is better able to withstand the trauma of an accident vs the damage it would cause if they were younger.
Here is an example..cervical vertebrae for a one-year old (left), and beside it a cervical vertebrae for a 6 year old (right). You can see at 1 their vertebrae is still in pieces:
thoracic vertebrae for a 1 yr old (left) and for a 6 yr old (right)
As they get older the body is continuing to fuse and develop..in general the older they are, the more likely they are to survive the trauma..hence being 12 or 13 or older before sitting in the front seat. Also the position of the head based on height/age regarding a front air bag deploying can mean the difference between some bruises and death..the air bag can catch the head/neck in the wrong position if the child is too small and result in a broken neck or other serious injury/death.
It's not like they don't test this stuff out and don't have any idea the height/weight and body positions that are necessary to reduce or prevent serious injury and death in the event of an accident. Like I said before..there is countless data that proves the usefulness and safety from a child safety seat and also data and crash test that show the resulting trauma if someone too small is improperly belted. There is lno data I have seen showing child restraints like a car seat/booster causing similar serious injuries..on the contrary they have been proven to reduce trauma/serious injury when used properly.