seems to me this alone is a show-stopper besides all the other terrible implications:

“the best estimates show around a 10 percent false positive rate for client-side scanning – which could see a huge number of people accused of crimes they didn't commit.”

10% is massive regarless, but of what? 10% of messages being false positive flags would mean almost everyone getting flagged within a day.

Upsetting statistic for other reasons: Even if it's "10% of all flags are false, 90% are correct", if there's also no false negatives, then the 10% false positives alone gets you to about the current total incarceration rate — offenders are estimated to be a few % of the population, prison population is about 0.1% of the total population.