The educational Red Queen Race that S.Korea and China have been famous for -- with childhoods utterly dominated by schooling and "extracurriculars" -- is now also an American thing. It's hard on kids, hard on parents, and the real educational benefit is less than zero. (In that kids would genuinely be better off without excess schooling and with more self-directed free time.)
The Ivy League path is simultaneously ultra-high-stress and ultra-conventional. It's no longer a golden ticket to the good life. Unless you like grinding and competition, and want a career which features both, I feel it's usually just the opposite. So it makes sense to "say no," as those in OP do.
Truly, kids aren't getting a "head-start" these days -- they're engaging in ever more complex and Byzantine make-work. And, emphatically, I do not believe it has any impact on their intellectual achievements later in life.