Completion rates are a red herring for a variety of reasons. The biggest is the total lack of risk inherent in joining a free class with no transcript. In college you should be more conservative about enrolling for classes because you can ruin your academic record or waste money, and you also often have degree requirements and schedule conflicts to worry about. With MOOCs you might as well join whatever you're even half-interested, even joining more classes than you can possibly realistically complete and finish whichever ones seem the most promising after a "trial period".

Generally speaking, for any system, reducing the cost of failure will make the failure rate go up.