These drugs are expensive, can have bad side effects, and success is highly variable. Some people lose a lot of weight (like going from 270lbs to 170lbs), and others much less (from 330lbs to 300lbs).
One individual only lost 25lbs and regained 17:
Alam said he takes the drug to help him control his blood sugar and shed extra pounds but has gained back about 17 of the 25 pounds he originally lost after starting the drug. Travel and returning to the office disrupted some of his healthy routines, Alam said, though he still feels more agile.
Pretty disappointing. Up to 15% of people on semaglutide lose little to no weight:
In a clinical trial , around 86.4% of participants taking semaglutide lost more than 5% of their body weight over 68 weeks, or a year and a half. The rest, or, 13.6%, did not — and it’s unclear what that percentage would be within 3 months.
Or discontinue usage if the drug does not produce enough weight loss to justify the cost and side effects. The data also shows that discontinuing these drugs causes rapid, v-shaped weight regain, shown below. It’s not like you can slowly taper off them and continue to get the full benefit; either you’re on or off.
Thus, even with mass adoption and insurance coverage, there will still be a lot of fat people due to the variability of success, discontinued usage, etc. It is not at all like waving a magic wand and fixing obesity for the general population. 
Some people eating smaller portions may mean smaller profit margins, but this is also offset by other factors like population growth and shrinkflation. McDonald’s makes a profit whether people buy big meals or small meals, and smaller meals have higher margins. ‘Happy Meals’ are one of the most profitable and successful items in the history of McDonald’s.
Additionally, weight loss treatments have a long history of losing efficacy over time, such as weight regain years after gastric bypass/band procedures, which is quite common. Or developing tolerance to weight-loss stimulants. Most people who are on GLP-1 drugs have only been on them for less than a year; it remains to be seen if the effect wears off over time, which would be a huge problem and could even lead to consumer lawsuits.
The body is really good at resisting weight loss and will do everything possible for as long as possible to try to return to its set point, sometimes many years after the initial weight loss, as the famous Biggest Loser study from 2016 showed.
At no point does the body ever give up trying to regain the weight. As powerful as these drugs are, they do not fix the underlying cause of obesity.
The real game changer is a safe and effective metabolism booster or stimulant, like DNP but without the potentially lethal overheating. Or phen-phen, minus the heart valve damage. It would be way better in almost every way. People would be able to adjust their metabolism with said medication like a thermostat to hit their desired weight. No company has been able to do this yet, but if it’s done, it will make GLP-1 drugs obsolete or redundant.
This is the main reason I have not invested in Eli Lilly and Novo, in addition to the possibility these drugs stopping working. Even a press release about tentative successful trials of a metabolism booster would be bad news for these stocks. Of course, this is way easier said than done. It necessarily follows from physics that raising metabolism entails raising core body temperature, but doing this safely is the hard part. Given the huge market incentives, we can be sure at least some companies are working on this.
Drug companies, unlike tech companies, do not have network effects. If a newer and better drug comes along, everyone will switch to the new drug; they won’t stick with the old drug just because it’s popular or their friends are on it. Investing in a biotech company is like making an implicit bet that a better drug by another company will not come along, which is not a bet I would feel confident making.
A metabolism booster would also be bullish for food companies, and allow people to eat full meals without the potential nutritional deficiencies and muscle loss of GLP drugs. Because GLP-1 drugs act as appetite suppressants, malnutrition will always be a risk. 
To get an idea of how big of a deal a metabolism booster is–and why metabolism is so important regarding weight regulation–consider cancer cachexia. Weight loss in the setting of cancer is mostly due to changes in metabolism, not treatment of the cancer, which is why many people with cancer lose a lot of weight well before the diagnosis. It’s the significant weight loss that brings the otherwise asymptomatic patient to the doctor and leads to a diagnosis. Despite substantial weight loss, it amounts to a net daily deficit of just 200 kcals/day:
The average calorie deficit in weight-losing patient is reported to be approximately 200 kcal per day in the setting of advanced cancer and 250-400 kcals/d in those patients with cancer cachexia. An average supplementation of 1 calorie/mL has not been shown to improve the nutritional status of patients receiving chemotherapy[140,179].
That’s all. We’re not talking that many calories, but it adds up.
 Insurance companies are hesitant to provide coverage of these drugs. Health insurers make a profit in one of two ways: by charging a fat premium for cheap drugs, like insulin for diabetics or statins, or by insuring rare events. Ozempic falls into neither category. It’s funny how people think that getting insulin and other generic drugs is a ‘good deal’ or justifies health insurance, when they are paying thousands of dollars/year in premiums for drugs that would be far cheaper if purchased without insurance.
 Wouldn’t a metabolism booster increase appetite to compensate? Not necessarily, at least not with DNP. People taking DNP lose weight in a predictable manner, increasing linearly with dosage. Metabolism is what happens after the food leaves the stomach. A fast metabolism does not mean being more hungry, as hunger is a sensation felt in the stomach. This is why asymptomatic cancer-related weight loss does not lead to increased hunger to compensate, but rather appetite is unchanged. Conversely, people with slow metabolisms doe not feel less hungry; rather, more energy simply gets stored as fat.