It's rude to single them out publicly, but it's absolutely fair to require additional payment if they can't fit in the purchased space.
If I purchase my ticket with the expectation that I get full use of the limited space I have purchased, I'm going to be furious if my arm has someone's fat flopping over it.
Yep. I flew all the way from Zurich to DC like that, and she was wearing some multi-layered flowing gown to boot. She was very unhappy that I didn't want the armrest up, and got mad when HER sleeve fell in MY plate.
What informed your expectation?
The only informed expectation I see in any documentation is that you will be flown from Point A to Point B in a seat.
You do realize that airlines have made the seats narrower and shallower, and moved them closer together in the past 10 to 30 years, don’t you? Why do you suppose they’ve done that?
If the obese person next to you DID buy two seats, you’d just want to park your magazine, newspaper, pillow and blankie on part of their second seat. And you’d want to put your drink on his second tray table. You know you would.