It seems to me you are mostly right but not all the way right. People eat until they are full and also many, many people have a broken sense of satiety. Of course part of the blame lies with foods designed to incentivize people to overeat.

However.

The average person is in my opinion inexcusably ignorant of what foods (s)he SHOULD be eating. This education topic is my wife's job, but the amount of people who can't tell you what a carb is, or as adults don't eat "green" foods, or a million other things is astounding. There's an education crisis on this front.
There are remedies to the problem of satiety (though surgical) but I suspect habit also plays a large role. You don't need to count calories your whole life to get the hang of portion size. Not everyone has the executive function to stick to it, but that's the next point.
We don't demonize people with broken arms, that is true. BUT people with broken arms seek treatment, and also they heal in less than a year no matter what. It's disingenuous to make being overweight for your whole entire life the fault of the rest of the world. There are absolutely contributing factors, but there is also only ONE person who is accountable. I'm not saying weight should be demonized or persecuted, but "it would be hard to lose weight" is a pretty dumb reason to increase your all cause mortality without at least trying a lifestyle change.

This last bullet point is why I'm suggesting the physics approach. It is irrefutable. If you put more in your body than you use, you will gain weight. If you put in less you will lose weight. There is literally NO WAY this doesn't work. It may not be pleasant or infinitely maintainable for everyone, but it's true.