I suggest that we should actually care less about whether in some abstract sense we can do these things ... and more about whether we will.

These are somewhat different concerns in the sense that "can" is not sufficient for "will", but it is necessary for "will". Since I cannot fly by flapping my arms, the question of whether I will fly this way doesn't have much meaning.

I suggest that, instead, we stop pretending that there are solutions suitable to absolutely everyone. People are different and are sufficiently different to require quite different approaches. If we take weight as the example, some people (commonly called "that bitch/bastard" :-D) can eat whatever they want and maintain weight; some people can control their weight purely by willing themselves to eat less; some people can control their weight by setting up a system of tricks and misdirections for themselves which works; some people cannot control their weight by themselves and need external help; some people can't do it even with external help and need something like a gastric bypass; and some people have a sufficiently screwed up metabolism so that pretty much nothing will make them slim.

There is no general solution -- it depends.