I've had people sitting in the window seat while it was mine several times before, but upon telling them, they always moved away. I don't see how this would get

I've had people sitting in the window seat while it was mine several times before, but upon telling them, they always moved away. I don't see how this would get worse with a rule like that in place. Sitting in the middle can be pretty bad if you're a tall person. I once had a ~10 hour flight in the middle with a morbidly obese guy sitting on one side, and a ~200cm tall guy sit on the other. It was torture. I'm 188cm myself, so I'll be pretty crammed either way. I somewhat envy my girlfriend in that way, as she's asian-sized and can comfortably sit anywhere. On my last flight with United, I got assigned a window seat in one of the exit rows. I thought 'great' as they offer extra leg room, but again, there was a morbidly obese person next to me, and he took up about 20% of my seat (or even more.) It was hard to breathe so I mostly sat on the bump of the emergency exit door. A flight attendant flew by and asked me if I was alright. I gesticulated to the guy (who was sleeping on an opioid pain killer mixed with Heineken) saying I can't breathe properly while being squished between a 300kg guy and the airplane window. She just shrugged and said there's nothing she could do as the plane was full. I understand it is probably uncomfortable to fly for "extra size" people, but I also feel like I should at least get the very limited space I booked when I have to fly. ... Not too related but yeah.