How could this type of blackmail happen if someone was already reviewing the videos manually? Takedowns being automated is fundamentally at odds with free speech.
YT is legally required to accept all takedowns and takedown counter-notices to reduce liability. Not taking down something in lieu of a DMCA takedown request would put YT at risk of a lawsuit if the submitter does own said copyright.
Content ID, on the other hand, is a problem of their own making.
