They'll usually _track_ course completion, but it's not necessary for a course to have a high, or even an average completion rate to be a successful course. From what I've seen, they'll track active time spent in course, how much of the course is completed, use quizzes to reinforce learnings while also getting a check in on how actively the student has been taking the course, etc. They also rely on ratings and qualitative feedback. It's not as precise as something like "completions", but from what I've seen it's reasonably effective at assessing course value.