I can’t speak to comparing private prep to magnet - we have no magnet high schools in our school district. But I can compare private prep to our local, supposedly high end public schools. There are loads of reasons to choose private prep beyond college admissions. We moved here for the public schools, yet ended up sending ds to boarding school - a choice much maligned by our peers. But for this kid it was the right choice- for lots of reasons. Kids in our great public high school are under immense pressure, spend loads of money on extracurriculars and counselors, have higher risk for drug abuse, and the school offers nowhere near the opportunities for exploration. And “free” education is far from free. The fundraising is non-stop.
DS isn’t a legacy, or urm or recruitable athlete. We couldn’t afford boarding school but for the help of extended family. I agree that ds has an advantage in getting into a fancy college now over his peers back home, but that is more about the ec and academic opportunities available to him, and the personal relationships he has with teachers, than the name of the school. His personal growth has been off the charts amazing. Two years in, I see the person ds is developing into, and don’t give a damn what college he goes to. Boarding school was the best parenting decision we ever made. So I take issue with the notion that there is no point to a private prep education if there is a great public option.
But that isn’t the question op asked. Yes I think there is a real advantage for college admission for elite private prep schools. But it isn’t just by virtue of the school name alone - it is the educational and growth opportunities they can provide. Don’t send your kid to one if all you care about is college admissions. They aren’t a golden ticket. Elite college admissions comes from your kid taking full advantage of the opportunities prep (or public) schools offer. There are just more opportunities at elite privates and they are easier to tap into once there. Ymmv, based on the specific schools available.
Edited to add: the diversity at most elite boarding schools blows our local public school diversity out of the water. That is huge for us.