That's not a valid argument, though. Firstly nobody lives forever. Second you don't have to exert all the effort at once, so the totality of effort doesn't matter. At any given time you just have to decide whether to have the snickers bar or the apple. And that's not an impossible effort. We don't live our entire lives all at once. We just have to be present for one moment at a time.
Edit: In my opinion it's hard for two reasons. We have cravings for high calorie foods. And no one candy bar will make you fat, so it's easy to think "I'll exercise more tomorrow to make up for this indulgence." But then you don't, because that's hard too.
> Firstly nobody lives forever
Lose weight permanently through cremation?
> At any given time you just have to decide whether to have the snickers bar or the apple.
No, you have to decide to even think about the difference between them instead of thinking about something in your life that feels more important. It's a sort of cognitive opportunity cost. You have to consciously think about food (instead of something useful) forever, because your body's instincts are telling you to do the wrong thing and you need your rational mind to overrule it.
So for the rest of your life, every day, until you die you must decide to stop and expend effort making that decision instead of thinking about work, family, politics, or writing a new bit of code that will change the world. Most human beings can do it for a while, but not forever. The only way to do it forever is to get your body chemistry on your side and reduce that cognitive load.