Well, the proposed solution works basically 100%, the problem is, as with most issues, people. Tell folks they should not eat sweet things for rest of their lives and half go catatonic when they realize reality of it. The other half go medicating such induced stress with a bucket of ice cream or similar. This is not problem of underlying solution per se, its the problem of expecting too much out of most people.

But you are right with the fact that at the end, it mostly doesn't work. But those who stick with regime, it works 100% and positive changes in life, quality, happiness etc are massive.

The real solution - treat underlying mental issues. They are always there with obese people, they are massive (no pun intended), and they manifest as obesity, among plethora of other mental issues.