I would never do this in Y but in FC I guess I am worried that I will be publicly shamed by the FAs since they are all considered "good" seats (I have actually heard that said by someone before when they would not move when asked). Also, when it's someone who wants to sit with their (even teenage) child, I am sort of afraid of a rude response by the parent and the FAs being mean to me!
But yes, it does encourage the practice, and I think it's rude to already occupy the seat befores asking. I think I have been too nice, but I'm not sure what a good response is as to why I must sit in my seat other than I prefer an aisle. That can come off as quite selfish-sounding to two people who want to sit together.
Although - after moving for selfish/rude people so much lately and that last bad experience I think I'll just have to say no - or else stop getting to the gate so late and get my butt in my seat first.
(P.S. CO DCA - That was not me! But I did once move 2x before. Ok, everyone stop asking me!!!)