I'm afraid I don't entirely agree with you. Disclaimer- I am not a car seat technician (yet!)

. But in all my reading, it seems the plastic parts are usually the parts that weaken, break down and fracture with age and use. Hence why seats expire in X number of years. I have heard many techs speak of plastic micro fractures being a possibility from seats being tossed or mistreated by baggage handlers. As for them being delivered to a store, this is why they are transported in heavy, corrugated cardboard. The box packaging is designed to act as a crumple zone to take any impact and protect the seat.
A vehicle seat belt does not *only* keep the body restrained. It is designed to be positioned exactly in the correct area to keep the crash forces against the hard bones of our collarbone and pelvis. A seat belt laying in the wrong spot against the soft abdomen can, and has, caused fatal internal injuries, especially in small children. So the booster is crucial to ensure the child's lap belt stays on her lap and does not ride up to her belly. If the boosters structure has been compromised by unseen damage, it could fail in an accident, putting the child in improper position for the lap or shoulder belt, which could absolutely cause fatal injuries.