Good points. The lack of scientific research discussed is certainly an issue. I did a quick literature sweep before writing this post, but decided not to include that information here.

One is a sunk cost fallacy. If you have sunk ten days into it you are less willing to ditch it because fallible humans are often unable to act like good economists and recognize that sunk costs are irrelevant.

At the dhamma.org courses I haven't found that to be the case. The management at the Massachusetts center informed me that a large majority of students never return to take a second course. Perhaps the cost needs to be larger; people may find it difficult to give up the practice (when they have good reason to) if they have done it daily for some length of time.