I've seen that happen once on a BA flight I was on. I was sitting in J and the pax didn't even bother getting as far back as their seats. They just saw empty space and shoved their bags in and kept going.
I think the real issues with cabin baggage stem from the failure or unwillingness of airlines to properly enforce their own policies. It doesn't help that fees for checking in bags and lack of confidence in airlines to get bags back to passengers promptly after the flight arrives contributes to people taking more as carryon.
Yes, I agree, the carriers allow bags as large as my checked bag into the cabin too much of the time. They make a rod for their own back, and the FA's hate it! I don't want the "baggage police" senario, but at check in, they can mostly see what you intend to take into the cabin, unless you are a "smartie" and have a friend keep your "other item", while you check in!
A number of years ago at SYDKT, BA at check-in, hectored me into taking a few small items out of my hand luggage and putting it into my underweight checked bag, to make the 7Kg limit.
I remonstrated with the service desk senior, who was rude and dismissive.
To make things worse, when I got to the boarding gate, I was further hectored by an officious flunky, who demanded I open my hand luggage and was checked again!
Naturally, I have never flown willingly with BA ever since, though they have changed now by allowing almost any amount of hand luggage!
So, yes, a little bit of sanity please with carry-on!