All research points to a "no" answer - weight is regained, and quickly. Which helps explain why obesity is so prevalent - it is something in the brain's chemistry.
People, on average, eat until they're no longer hungry. Problem is, there's only a loose relationship between your caloric needs and your hunger response. That's how you end up with underweight people who are trying to put on muscle saying they can't possibly eat any more and still can't put on weight, while having overweight people who eat twice as much as that guy and have to actively choose not eat more. Both people can make a conscious choice to disobey their signals, just like how you can choose to hold your hand to a hot stove. But it takes a lot of energy to keep up that willpower. Effective weightloss drugs solve that problem, by treating the actual problem: the hunger.
Speed of eating might also be an underrated factor in all this. Stretching out ones meals and slowing down the pace might lead to satiety triggers coming before the meal is done, whereas if one scarfs down the plate before that signal happens, well, one already scarfed down the plate and might be working into the next before those signals hit. This meta analysis suggests this is a possibility (1).
This would be true if not knowing what an appropriate portion size is was the one thing keeping most people from losing weight. If that was the case, traditional dieting would have a far better track record with long term weight loss.
Australia's health organization did a meta study on people who had bariatric surgery. They found that every single one regained 70% of their original weight after five years, even though they were physically incapable of eating the way they did before.
This happened to my grandmother, she had a bypass in the 2000s, lost over a hundred pounds, and then regained it again and was back to her original weight when she passed in 2022. The woman couldn't eat more than 4 ounces per meal without throwing up.
I lost 40 pounds in 2017 from gastritis. I kept it off for three years, and then regained 50 pounds despite starting ozempic.
And you mention exercise, that is an excellent point. Hunter gatherers might forage for 8 miles a day, while many peoples daily walking effort can be measured in a few dozen feet. Our bodies are built to be used. It is no surprise that when they are not, systems designed for a certain baseline load are no longer functioning as intended.
(they do they're fucking called glp-1s)