I have a B&R rolling computer bag and a small backpack. Both of these together take up less room in the OH bin than my 21" roll aboard and I can fit both of them in in one "row" if you will, taking up no more room than the roll aboard. Ive never had a problem at all. I can even shove a jacket or blanket on top of the computer bag if need be.

As far as I'm concerned, if they really want to enforce what bags are in OH and what are under seats, it begins with which bags make it on the plane. Both of mine will fit in the sidewise checker, so I have zero guilt as I am following the rules.

ETA: If the policy changes and it is only 1 item in OH, I will switch and always use my larger roll aboard and therefore take up more room than I am currently doing. I'm 6'3" if that actually matters.