As the main problem for the "body positivity" I see is that it still matters a lot. It's not like the pendulum has stopped in the middle, but went from fat-shaming to fat-cherishing, ignoring the health you mention. Sure, fat shaming wasn't healthy, either, psychologically and physically in the aftermath. But instead of preaching mere acceptance, shifting the focus from looks to health, it was kept on looks, just judged differently.
The interesting thing might be, that it was healthier than today. People were on average psychologically and physiologically healthier than now. It isn't just the shaming part of course, but the whole direction of culture that has got us to this point and if we don't do something, we will keep going down this path to oblivion.