First, it just seems like this kind of "parental optimization" is likely to have such a small effect on long-term outcomes that it's probably a wash overall.
Second, kids don't acquire all their information about themselves from their parents and teachers. By the time they reach early elementary school, kids, especially smart ones, know whether they are trying harder than their peers and whether they're succeeding more than their peers. They know what was easy and what was hard from their own experience, and they have a theory of mind and if an adult tells them something at odds with their experience they could just devalue that adult's credibility.
Third, when it comes time to start thinking about what you want to do for a career, it really pays to choose the things where you can succeed without feeling as if you worked very hard.
Finally "hard work", it self is a vague concept that I think often misleads young people into to thinking that effort is not merely a means to an end, but the end itself. In fact what we really mean by "hard work" is some combination of traits like diligence, conscientiousness, and persistence that lead people to dot all their 'i's and cross all their 't's, and not quit until they've succeeded. In fact, when you become an adult and are out working, nobody cares how hard you worked, only that you succeeded.