Bodily autonomy means people can make choices about their bodies that you think are bad.
If you want to stop women from using cosmetic surgeries to fix their insecurities. Fix the culture that created those insecurities. Blaming the existence of surgical procedures is stupid, solves nothing, and hurts a lot of people along the way.
Do y'all think there's a clear difference between a medically necessary and cosmetic surgery all the time? If a firefighter gets half his face burned off saving someone from a house fire, a plastic surgeon can help reconstruct them. Don't y'all think the happiness and self-esteem of that person is a major reason for the surgery?
Sure a doctor could be like "Ehh, the firefighter can still eat and talk all right, so it doesn't matter what they look like. They only feel bad about having their face half burned off because of negative social stigma." And you know what, it's true, it probably is because of negative social stigma. But if the firefighter wants to have reconstructive surgery and is informed about the risks, it's important health care and probably gonna improve his life.
"Well, having your face burned off is obviously different!" All right, what if you're born with it? I have a congenital condition called pectus carinatum (or "pigeon chest") that causes my sternum to jut out. The internet calls it a deformity, but I think that's a little bit rude. Quick, would correcting this be "cosmetic" or not?
Turns out it's not quite that simple, and whether or not a condition needs treatment has a lot to do with how the person feels about it. As it should be!
It's impossible to be "against cosmetic surgery" because "cosmetic surgery" is not clearly, unambiguously defined.
How about somebody who wants a breast reduction because of their back pain? How about somebody who gets a double mastectomy because breast cancer runs in their family? What about someone who is sick and tired of having big breasts for reasons that are their own? What about a breast cancer patient who wants breast implants after a double mastectomy? What about someone who wants breast implants because they want bigger breasts? What about someone who wants a hysterectomy because of their endometriosis? What about someone who wants a tubal ligation? Which of these people have a legitimate medically necessary concern and which are being manipulated by an evil misogynistic medical establishment?
Why assume that any particular person who alters their body is insecure or being manipulated, anyway? Capitalism is horrible towards women for making them feel their bodies are repulsive, but body modification is a hell of a lot older and it's damn near universal among human cultures. Go outside and you'll walk past people all over the place who have lip piercings, tongue piercings, eyebrow piercings, just all kinds of holes punched in them. People dye their hair all kinds of colors and get tattoos of everything from flaming skulls to Tweety Bird. In a perfect world, some people would want to alter their bodies just because it's possible to do it.
Why do people get breast implants? I don't know, why do people get tattoos or ride horses or have children or go to the Moon or jump out of planes or devote their lives to accruing the world's largest collection of belly button lint? Why DID people in the 80's think those hairstyles looked good?
If a woman is getting cosmetic surgery because misogynistic society has made her hate her body, the problem is misogyny, not the fact that surgeries that alter your appearance are medically possible.