Fat people won't miraculously disappear just because reliable weight loss drugs exist. Between limited supplies and pricing, as well as U.S. healthcare being dogshit in general, I suspect most people just won't have access to ozempic and similar drugs in most cases. If getting ozempic in the near future is as difficult as getting actual necessary-to-survival medicine right now, it'll be an absolute bitch, and even more so if it gets categorized as "non-essential" as it likely will if it isn't already, thereby ensuring insurance companies won't cover it even at gunpoint.
Also, don't forget that less fat people does not equal the end of fat fetishism. Even back when food was scarce at various points in history and being obese was a rarer luxury, there were still plenty of people who were turned on by fatties and would seek them out when possible. And just as there will still be fat-lovers, there'll still be men and women who enjoy being fat and getting fatter. Unless someone finds a way to permanently fry the neurons in the brain responsible for fetishes, they're not gonna go anywhere, and if that happens we'll have way bigger issues to worry about.
In the "worst case" scenario, where ozempic becomes piss-easy and cheap to get and the average person is on it with no ill effects, all it'll do is weed out the accidentally/unintentionally fat folks who don't enjoy it, leaving only the FAs and willful BBWs/BHMs. If you ask me that's a net-positive, only gets better when both people in a relationship are into it and willingly participating, and no good relationship I've seen has ever been founded on lies or deception like secretly-fattening partners. If you're the type who can only enjoy it when it's forced or sneaky, roleplay exists for a reason.
Let the thin people be thin, if it works out that way, will just make it easier to get a fat girl or guy who'll appreciate the attention and taste for their fuller figure.