If people can't or won't take their medicine, is it the medicine's fault?

When people lack the intelligence, willpower, or consistency needed to make prescriptions (of pills, exercise, or diet) work, there's not much you can do for them except lock them up and force them to do the things.

If you need to be on a diet and you can't stick to it, there's not much we can do for you except maybe come up with a pill that makes your brain hurt less when you're on the diet, like we do with curing drug addiction.

Yes, if it is not possible for majority of humans to take medicine as prescribed in long term then medicine is wrong.

What you can do in that situation is to try to give them different advice and improve advice untill it actually works.

There already is a pill like that (well, at least one: phentermine), but it has side effects which are dangerous for some and unpleasant for some.

There may be tricks to help people stick to a diet or exercise plan, but you won't find them by just calling it "user error".