The issue has never been with how it could potentially make things better. We could strip ourselves of some particularly nasty genetic diseases, for instance.

The trouble has ALWAYS been that literally nothing is sacred or beyond corruption. You will put the infrastructure in place to sterilize people with some awful genetic disorder, and while it's sad, you could save the future forevermore from it. The next guy in control however suddenly has everything in place for much darker things.

I call it the "scaffold theory". You can build it to reach the shiny sunny places, but if there's dark stormy ones at that height you've just enabled anyone who wants to take that road without any means of stopping them once you lose control.