I myself have a Lap-Band. It is a silicone implant which can be adjusted with saline injections according to how the person is losing weight, so my rate of weight loss is essentially under my control. That’s one of the things I like about it.
However, I suspect you are thinking of the gastric bypass, which is the more common operation in the USA right now. This is how it works for that kind of operation:
In very rare cases (people on the lower end of obesity who get a lot of intestine bypassed), it IS possible to lose too much weight with gastric bypass. For such people, reversal of the surgery might be considered. However, reversal is just as risky as the original surgery, so it would only be a last resort.
For most people, there are several factors that keep them from losing a dangerous amount of weight.
With gastric bypass, the “window of opportunity” for weight loss is about a year post-op. After that point, the intestines adapt to being bypassed and start absorbing more calories again. Plus the stomach does stretch a bit.
Plus, even though it sounds counter-intuitive, fat people actually burn more calories than thin people do. It’s hard work for the body to lug around 100 pounds of excess weight, so an obese person will burn more calories during physical exertion than a thin person will.
That means that, if you’re eating a reasonable amount of calories, you will eventually get thin enough that you aren’t burning as many calories as you’re taking in. Then your weight stabilizes.