Indeed. Also some platforms (I believe Coursera and EdX were like this last time I used them) only let you access old content if you signed up with a course when it was active. So you are incentivized to sign up for any course if you think there's a possibility that you might want to access some of the content at some point in the future.

People shouldn't confuse completion rate with a particular course with a student reaching proficiency in a particular subject. There have been several courses I've started were I quickly got annoyed by the copious amount of filler in the lectures and dropped the course in favor of a book or web tutorial.

In general I think MOOCs are doing themselves a disservice by trying to copy college courses.