That attitude guarantees failure in the long term, even if they actually kept a strict diet+exercise plan for a while (which they never do). The same thing exists in other ways, like someone who says "I worked hard at work yesterday so I won't show up tomorrow" - and then can't understand why they get fired for being a shitty employee. When I do a heavy workout I actually am super careful about what I eat afterwards, so as to not ruin the effect of all that hard work. And it motivates me to do even better next time to build off of it.