The world is full of people who were unable to maintain a diet. If you can't maintain a diet, then that diet doesn't work.
No, that means that you can't maintain a diet.
If something fails for lack of maintenance, it doesn't work. It's kind've self-evident.
The diet didn't fail. The person failed. There's a pretty big difference. If someone's diet plan consists of "don't each a cheesecake every week," and they continue doing that, you don't say "this no cheesecake thing is nonsense, that doesn't work!" You wonder why that person doesn't just stop eating the cheesecake.
Next you'll be telling me that the US won the Vietnam war. I mean, the politicians failed and the military failed, but the plan would have worked if they could have seen it through. Therefore the war worked.
So if my car fails because I never change the oil, cars don't work?