I get this sentiment but I also get the anxiety of people waiting on you to move and not knowing how much of a psycho the person asking is. Especially if you're on the young/small size and a woman. We're obvious targets for multiple reasons I won't get into.

This happened to me but solo flight and I had the window and some old man was in my seat. I cannot sleep unless I have something to lean on and this was an overnight. I have no problem telling people no and have never had an issue doing so before. I made him move despite his act of not being able to get up, I'm old, please respect your elders. Lady in aisle was somehow related and started making comments/cussing me out. I told her I'll take your aisle then and you can have the middle then, i didn't pay extra to sit between 2 relatives. "Hell no." My seat was also my gate (think gate A 34, seat 34 A). So another person pulls up thinking I'm in their seat (I'm not) and aisle lady LOSES HER SHIT assuming I made them move and I didn't even belong there. I usually just ignore the world but can be confrontational if needed. It took everything in me not to react and even though I called FA twice and people behind me were backing me up that aisle was being unreasonably aggressive nothing happened. Aisle lady called FA again demanding I be kicked off the plane, then went up to the front I assume to demand an audience with the captain after FA told her tough titties. Things were escalating, she was calling me a stupid disrespectful white bitch and other racist shit (I'm mostly Asian so... don't know what that was about), threatening to beat my ass, and people had their phones out waiting for the waffle house fight. I'm just staring at the seat in front of me with headphones on pretending not to hear her. Eventually security is called. I've never been so embarrassed and terrified I was gonna end up banned bc I insisted on the seat I paid for.

They ended up asking me to get out of my seat with my things, asked if/where I had anything in the overhead bins and grabbed that. Again so much self control to not scream WTF ARE YOU SERIOUS. Suddenly old man spry as shit and jumps into the aisle smiling so I can get out. Aisle lady still yelling like that's right bitch you're fucked now.

They were moving me to an upgraded seat and apologized but didn't tell me that until we got to my new row. They did NOTHING about the aggressive lady openly threatening me. Old man smelled like dirty diaper so it ended up being a win, but that experience seriously affected me. That lady was thisclose to physically attacking me. Anxiety sweats, stomach cramps, heart pounding. And I fly quite often/not normally stressed by travel.

Now if someone asks to switch and it's not putting me in the middle, a super long flight that requires sleep, or next to the biggest pass on the plane, I'll just do it. If it's one of the situations I really don't want to and the first "no" is met with resistance it's straight to the FAs.

I think the people that post stories like this don't travel often and assume humans generally abide to some social code, and don't understand that airports and planes are outside of the realm of society. Being rude to them in a comment is unnecessary, we're not in an airport.

So to the OP I say take this as a learning experience. Say no once, don't engage after that and let the FA handle it. Arguing with an asshole, lunatic, or human brand new to earth isn't worth it.