In Y? I'm a window seat flyer, but I never noticed any issues with the way the space is allocated when I occasionally end up with an aisle seat. In F, that's another story. I agree that if you have a legal personal item and it will not fit beneath the seat in front of you, you should put it in the overhead, but I don't think I've ever seen that with an actual, legal personal item. I'll have a lot more sympathy for someone who makes an honest effort to use the space under the seat in front of them and then has no choice but to put their bag up than I will with someone who thinks they're entitled to take up OHB space even with an item that would easily fit under the seat.

It simply amuses me to see others' transgressions used as justifications for one's own. I agree that UA should do a better job of stopping people with oversized / too many carry-ons before they get onto the plane, but I don't use their failure as an excuse to be an OHB glutton myself. (And this goes doubly for the people who have openly admitted to putting both bags in the OHB as a matter of course).