In the US they let 22x14x9 inch bags on as hand luggage, they use the same planes, have a faster turnaround, and it seems to work just fine.
I don't think you've been on the same aeroplanes as me then!
Enormous carry-ons blocking the exit rows, last pax on get their bags taken off them 'cos there's absolutely no locker space for the "tail-end" Charlies.
I defend the CAA's cabin baggage policy unreservedly, as would anyone who has witnessed the after-effects of clear-air turbulence. I witnessed this a couple of years ago when a returning flight from Alicante landed at LGW - blood everywhere!
Without exception, all the injuries were from Cabin Baggage spilling from the overheads - God knows what we would have seen if we had the US allowances!!!