This might be somewhat off-topic (I did not read the whole discussion above) but, in my opinion, preparing for the SAT/ACT allows the student to fill some gaps in basic grammar and math which our otherwise highly-ranked public HS somehow forgot to teach them. Maybe the private schools teach grammar but in our public school districts I see graded papers for grammar for the first time in the AP lit class taken in senior year.
My D18 was editor-in-chief of the school’s award-winning newspaper, a voracious reader, and really talented creative writer. Turned out there were some grammar rules that she did not know! After prepping over those rules for a couple of weeks, she got the max score in the reading/English sections on the ACT. I am sure, prepping for the math section helped even more to solidify her quantitative skills.
My S21 who reads far less than his sister benefitted even more. There is nothing wrong with prepping for these tests as this ensures a minimum baseline in math and English. So, regardless of who these tests helped or not in the college admission, I think they are kind of useful.