If any airline were to start flying without seatbelts, they would surely gain at least a 10% added revenue; not only from the savings in equipment and weight, but also adding to increased interest from the flying Joe public, to whom the standard safety measures clearly constitute a nuisance. And yet, not even the cheapest of LOCOs have used this strategy, and apparently no major airline whatsoever since about nearly 100 years. So it works to require a measure of added safety, even if it appears unpopular, to the airline and the populace. If it is a matter of "well, it makes sense to use a seatbelt, because in an extraordinary circumstance, it may prevent injury to yourself and other passengers and crew (in case of hitting the ceiling and get tossed onto someone else)". Why then is it constantly said in this and other similar threads that the same will never work for restrictions on the size of carry on luggage, even though the precise same reasoning (apart from the ceiling thing) applies? Oh, you say that there is a SARP that covers the seatbelt rule, so that airlines are forced to obey it, but there is no SARP for huge chunks of in-cabin baggage. The Standard And Recommended Practices of the Chicago Convention are actually subject to amendments. So would that not be the right place to start?