I don't think this is true whatsoever, most of the individuals I've coached, debated alongside, and debated against who've left policy due to speed have done so because they found themselves unable to compete and unable to do the required work. That's just anecdotal though. The people who thought speed was ridiculous didn't even do cx to begin with, and usually weren't very good at their chosen debate disciplines.
Additionally, your claim that "they would be better off educationally doing some other activity than debate because it doesn’t prepare them for anything but college debate, and then nothing at all" is absurd. One of the only aspects of high school that prepared me for college proper (not college debate, I don't debate anymore) was policy debate, specifically circuit-style policy debate. Although I debated in Missouri where "traditional" style predominated, I found this style extremely boring and associated with arbitrary, ill-informed judging. Those individuals who "survive" this "extreme" type of debate are oftentimes the hardest working, toughest, most intellectually capable debaters out there, because yes, speed is hard.
And by the way, I did adapt at tournaments like NFLs, because I liked to win. But those slow rounds in front of inexperienced judges were simultaneously soporific and torturous, like having a root canal in slow motion.