Sorry I've been meaning to post here.Yeah it's been a thing for a years and it's also been the plot for a lot of movies. I'd say it's mostly just lazy writing at this point since it's become a trope.
I know the overpopulation bit is very common. I even remember seeing comments about the world being overpopulated on GAF.
In the Thanos example, I'm leaning towards no obligation. I've been hearing about overpopulation being a big issue for years.
In the latter, I'm not so sure.