Interesting. I was not aware of that. I will have to check the next time I fly with my rollaboard, as it's under the legal dimensions. I don't have any evidence, but I would not have suspected that there would be enough height from floor to the bottom of the seat in front of me to accommodate a bag within the legal dimensions.

But, back to the OP, I have never been under the impression that I had any right to keep a specific bag in the cabin. I've always packed with the expectation that my bag could get pulled for gate checking. That's why I follow the advice to never pack anything fragile or valuable in a bag that couldn't fit under my seat. Same with prescriptions and other stuff that I would never want in the cargo hold.