Cabin Baggage issues.
Some years ago I made comments to the Baggage Management people of an American Carrier which transferred their pax onto our flights. Our carrier and their's had different cabin baggage allowances. The response was that their allowance was high for competitive reasons and if their over head could accommodate. If we didn't allow the same then it was our problem. So it's competition first and passenger safety second, although that would be denied.
The "Fragile and valuable" escape is a scam too. Define "Valuable." To me, a £200 camera is valuable but to a wealthy bloke, it's small change.
As I mentioned in my original posting which wasn't published in Pprune, some US carriers say at if you can't lift your bag into the overhead and it's too large to fit underneath, then it gets checked. I am not a gentleman when watching a woman struggling with her bag, then turning and looking pathetic. Your problem lady.