Failure to comply with the post op diet might be fatal within the first few months of recovery (although I haven't heard of that happening, I'm sure someone has died from eating too much), but after that, you are going to be mostly healed up. And its the long term that will determine if the weight loss sticks. It certainly isn't just because the surgery selects for people with willpower. Because almost everyone who has it has gone through years of losing and gaining back their weight over and over.
Bariatric surgery usually works for 3 reasons. First and most obvious is that it reduces your stomach size so you simply can't eat as much. Second, in most cases, reducing the surface area of the stomach reduces the amount of hunger hormones that get released thus making you less hungry all the time. And the third is malabsorption. Meaning that of the food you do eat, you absorb less of the calories than you did before. So if it were possible, you could potentially eat the same amount as before but still lose weight.
Bariatric surgery usually works for 3 reasons. First and most obvious is that it reduces your stomach size so you simply can't eat as much. Second, in most cases, reducing the surface area of the stomach reduces the amount of hunger hormones that get released thus making you less hungry all the time. And the third is malabsorption. Meaning that of the food you do eat, you absorb less of the calories than you did before. So if it were possible, you could potentially eat the same amount as before but still lose weight.