Your conception of fatphobia is distorted. Fat people are routinely denigrated, viewed as less valuable. They are paid less because they're fat. They're told over and over that they are unattractive. They seldom see fat people represented as normal people living fulfilling lives in media, film, and entertainment. Fat people are exactly like any other marginalized group: unfairly denigrated and devalued for reasons having nothing to do with their merit as human beings. That's what fatphobia is.
I disagree that the main reason people don't want to be fat is health. That's part of it, certainly. But at least as important are the social costs of being fat in a fatphobic society (see above). There is simply no way that teenage girls are going "OMG I can't be fat because I am deeply concerned with my long-term physical health." Teenagers don't give a f**k about long-term anything. What they are saying is "OMG I can't be fat because then I'll be gross, no boy will want me, and the other girls will treat me like garbage." Fatphobia is the driving force. To the extent that it relaxes, we will therefore see an uptick in fat young people.
There's a lot of hypocrisy in the health discourse anyway. Motorcycling is an extremely dangerous activity, 40X more likely to have accidents than cars; how come we don't have some big cultural push to abolish motorcycling, and why do we not find motorcyclists inherently unattractive because of their "unhealthy lifestyle?" Our culture is totally fine with some unhealthy behaviors and not others. So I think we need to look elsewhere for a full explanation of fatphobia.