My wife, a former stewardess for a European carrier, just suggested that this might, in part, be about safety. She thinks that decreasing the thickness of the luggage, but not the other dimensions to any significant degree, suggests that the European carriers may have been pushing toward underseat stowage of the carryons, which is much safer than the overhead bin. Basically the overhead bins are too flimsy to keep luggage from flying around. She also believes that this is a follow-on effect of charging for checked luggage. Anyway, not arguing against anyone's position, but her theory made sense so I was looking for a good place to share it.