That gifted students will be bored and not develop to their fullest extent, and perhaps more importantly, not develop the discipline to work hard in their formative years, is an argument for tailoring education to the student's ability. Asserting the opposite doesn't make it so. You argument comes down to impairing the education of gifted children in favour of improving the education of ungifted children. That is an argument if your top priority is equality of outcome, but one can question how wise that is if you care about the well being of all of humanity given the fact that it was a handful of geniuses who were responsible for a hugely outsized amount of progress.