As someone whose weight (like Echo's) crept up slowly over a period of years, and who lost a fair bit of weight around the age of 50...for health-related reasons more than appearance (creeping hypertension and some other issues) and who then worked in the fitness industry for a while...with respect to set point, I can only say: it depends.
People's metabolisms differ. You can shift your metabolism via exercise and weight loss, to an extent -- and if you do, then your set point may also change. Yet people's body types also differ (quite apart from metabolism), i.e. some people are smaller boned and some larger boned and some natural mesomorphs and some natural ectomorphs or endomorphs and that affects how much weight they comfortably (and possibly, healthily) carry and how easily they lose or gain.And fat cells want to fill up with fat, so once you have those fat cells, it can be more difficult to maintain lower body fat. And, as others have suggested, women in particular may not have a set-point because they may have been yo-yoing with their weight all their lives...either due to pregnancy, illness, dieting, or other factors.
Some people look at a bowl of ice cream and it goes straight to their hips. Others eat buckets of the stuff and it does not affect their weight (it may affect their health in other ways, of course.)
We tend to take our own experience as fully representative simply because it's the experience we know best. But what works for one person won't for another.
Anyway, with all that said -- I have in fact managed to maintain for about a decade -- more or less. (I went up by about 10-12 pounds during a period of very high stress but then lost that weight again. I'm now about 5 to 8 pounds heavier than my lowest weight -- which is probably right where I should be from a health perspective -- and, not coincidentally, more or less the weight I was in college. Is that my "set point"? Maybe.
How have I managed? Not by calorie counting. I don't even have a scale. I used to weigh myself once a week at the gym, but due to COVID I am not going to the gym -- so I tend to judge where I'm at from the fit of my clothes. I don't think about macros. I simply eat mostly veggies and plant foods plus lean protein and healthy fats. With mostly whole grain or veggie carbs (e.g. potato, fruit, sweet potato). Having said that, I ate regular bread tonight at supper and had leftover pasta for lunch. So, you see, I am the farthest thing from strict about this.
I move a lot (walking, biking, hiking, weights). I rarely eat prepared foods, but I drink wine, eat a lot of dark chocolate, eat dessert if I want to, and I'm sure my portions are sometimes larger than might be ideal for someone my size. (Rachy's points about portion size are right on, by the way. In North America, we tend to eat massive portions. My home portions aren't restaurant sized by any means, but they're not tiny, either.)
Would this work for everyone? NO. I must have a relatively high metabolism. Also, I love food and love to eat, but I'm not really an emotional eater and I don't get cravings, so I tend to eat when I'm hungry and stop when I'm full, no matter what is in front of me. Some people find that well nigh impossible and even if they could do it, they still might not lose weight due to their metabolism.
And if they aren't unhealthy, I say, why should they? I'm glad to see the new guidelines for doctors coming out in Canada on this subject. It is wrong to assume that a larger body is an unhealthy body. Some people are healthier at higher weights! We should all aim for our healthiest weight, not our skinniest weight.
Whether you want to lose weight, gain weight, maintain, gain muscle, or something else, it has to be a lifestyle change -- the changed eating or movement patterns. If it's a short term "diet" or "exercise program" the weight will just go back on when you stop the diet or the program.
