There are many gifted students and adults who aren't doing well.

Most of them aren't.It is unlikely that a gifted child will achieve to his potential later on if he doesn't experience any challenge or need for studying through grade 12.It's better to study and develop talents early.Most people left at grade level develop anxiety, depression, or give up on traditional education after 12 years of hell at the highest end of the IQ scale.It is very avoidable with gifted and accelerated programs.