It's irrelevant whether the bag can be shoe-horned into the OH. What matters is whether it meets the published and contracted size limits which are all the sizer measures.
If everybody knows the rules and the rules are enforced as to all pax, this problem disappears. And, we all benefit.
It now takes UA 40 minutes to board a 319/320/737 at DCA. Any European carrier can accomplish that in 15-20 minutes. The major difference is that there is no tolerance on hand luggage. People board, they sit down, the aircraft departs, arrives and we're off to the races.