The reality is that many travellers DON'T accept a simple 'no, sorry', or do expect someone to move into a lesser seat, such as from a bulkhead aisle in UA E+ to a middle in E-.

I 'own' 9A on a 757, 6C on the Airbus twins, and 3C on a 737 on UA aircraft. I book my flights based on that seat availability, even last minute.

I cannot tell you how many times (at least once per month) I have been shouted at, bullied, called names, and otherwise harrassed by travellers who feel that they have more of a 'right' to my seat than I. There is no competition between needs, wants, and having children, but some people do seem to feel that their needs trump anyone else's needs.

My habit is to board on the magic carpet, close my eyes, and put on my iPod, and doze. I cannot tell you how many people feel the need to physically shake me awake to demand my seat. That is the sad reality unfortunately.