> The real failure mode is much more mundane: People give up.

The other major failure mode is similarly boring: sometimes it's just not useful to blindly memorize things. The exact problem that spaced repetition solves is pretty niche, and while it can be "close enough" to be beneficial in adjacent endeavors, it's also extremely subject to Goodharting. The more hardcore you go into optimizing your SRS procedure, the likelier it gets that the metrics you're optimizing for depart from your actual goal.