HuffPo is BS in many ways, but they knocked it out of the park with their 2018 article Everything You Know About Obesity is Wrong.
It delves into most of the points raised by the OP and takes it one step further by investigating how many health issues attributed to obesity are, in fact, often byproducts of the stress of being fat and not the weight itself. While this commonly includes conditions like depression, anxiety, and disordered eating, it can also translate to heart problems as a result of the body producing more of the stress hormone cortisol, which can increase blood cholesterol, triglycerides, blood sugar, and blood pressure–all common risk factors for heart disease. Cortisol can also promote buildup or plaque in the arteries. Some studies have started to look at the life expectancy of those who are stigmatized and bullied for being fat vs. those who aren’t. Spoiler alert: people who are tortured and traumatized for their weight don’t live longer.
And none of this even scratches the surface of medical bias and the subpar treatment many fat folks face when seeking medical advice for something completely unrelated to diet or nutrition.
If people focused less on stigmatizing fat folks and wailing about health while also upholding systemic barriers that prevent fat folks from being able to participate in fitness activities in the same way as their thinner counterparts, a greater number of people might find an active lifestyle more accessible to them, thus promoting better exercise habits and overall strength and fitness. Many, many people, including those medically considered “overweight” or “obese,” enjoy being active–whatever that might look like on an individual level–and participating in activities such as organized sports, but avoid doing so because of fear of ridicule or stigma, or various barriers that are systematically placed in their way.
For instance: do you know how impossible it is to pursue a hobby in long-distance cycling while fat? How about kayaking? Surfing? Or equestrian? If you think any of the clothing or equipment necessary for these sports–such as cycling jerseys or one-piece suits, wetsuits, actual kayaks, or breeches–are easily accessible to people over a US women’s size 10/12, think again. And that is to say nothing about how fat folks can be ostracized or bullied for even trying to participate in fitness activities, even ones as basic as going for a jog or hitting the gym.
While thinness isn’t an indicator of health or physical fitness, the same also holds true: fatness isn’t an indicator of lack of health or physical fitness. They aren’t mutually exclusive concepts, but fat people are often barred from fitness or active spaces without experiencing some form of alienation or barriers to their participation, whether social, economic, physical, or otherwise.
(And none of this is to say that anyone, especially fat people, owe others fitness or “good” health, because that is how we slide firmly into ableism.)
We don’t owe the world anything period. Fatness doesn’t preclude us from being able to enjoy the same happiness and acceptance as thin folks, and we sure as fuck shouldn’t have to participate in health or diet culture to “earn” respect or acknowledgement of our basic human rights.
For those who are interested, there is also a fantastic article from Medium that looks at The Bizarre and Racist History of the BMI that I highly recommend.