Even if you override the natural instincts, you have to replace them with some other form of behavior. There is no ideal set of behaviors, unless you strip all humanity from a species.

Once you overcome the 'hurdle' of 'undesirable' behaviors, you just adopt a new set of behaviors, which could be, in the long run, just as detrimental as the ones we have now.

I think Brave New World describes this quite well. The world where everyone is happy isn't necessarily the one that wouldn't be shocking to us in its behavioral patterns.