In one of the highest followed and most completed course, which happens to be the first Machine Learning by Andrew Ng, the level of completion was a meager 12.5

In one of the highest followed and most completed course, which happens to be the first Machine Learning by Andrew Ng, the level of completion was a meager 12.5%. And in that Ng's course is an outlier as no other courses with similar sizes, with the exception of AI, have come anywhere close to that level of completion. The numbers actually show the opposite, a large course has a lower percentage of completion. IMHO if course community were a large factor this high trafficked course with a high level of community discussion would most likely have exceeded this. As you say later ML courses have had much community discussion, yet they have had lower completion rates. At the moment there is no substitute for real interaction with the docent or professor.