People _register_ for a course, presumably, with the goal of finishing it. (Otherwise, why not do a google/wiki search for topic X? Find that specific topic on youtube/MIT OCW?)
When the vast majority of people fail to complete the goal, you decrease their confidence in the process. Unless someone starts with strong academic confidence, they may be demoralized by giving up/failing a course. We shouldn't be happy that 5% of people enjoy high school math class if 95% of people graduate hating it, and never touch the subject again.
We can move goalposts and now pretend that MOOCs primarily exist as a random buffet of knowledge and not replacements for actual courses, but that's not how they were positioned.
"That pill I sold you, turns out it's no better than placebo... but placebos are 30% effective, so it all worked out, right?"