I agree with you, one of the contributors to the systemic problem is fashion (and by extension all media, print, digital, video, etc.)

My point here though is that it’s only a minority who “can’t help it, they were born that way”. Most would be able to overcome this issue if the dynamics were different.

I don't think that is true. Part of the problem is that once you are obese then it's already kind of game over. There seems to be a pretty strong consensus that losing large amounts of weight is extremely hard and even when it does happen, people tend to eventually gain it all back.

I see the body positivity movement as just trying to reconcile that reality. If you are already obese then you might as well try and find a way to live a happy life because you are probably never going to be slender again.

On the other hand that may be a deeply defeatist attitude and could further decrease chance of recovery

You just have to look at the statistics to see it's more about being realistic than defeatist.