Courses from edX, the MOOC provider developed by Harvard and the Massachusetts Institute of Technology, have a similar student profile.

A second problem is that when MOOCs replace traditional courses, an extremely high number of students fail. A well-publicized experimentbacked by Gov. Jerry Brown of California at San Jose State University flopped. In one of the MOOCs, just 25 percent of students passed; in another, only 50 percent passed, much lower rates than for the on-campus equivalents.

Those MOOCs were provided by Udacity, whose co-founder, Sebastian Thrun, is often credited with popularizing the massive courses after he opened his artificial intelligence class at Stanford to the world in 2011 and 160,000 students from 190 countries signed up. But what he ended up discovering was that he had designed MOOCs for the type of self-motivated, bright students he taught at Stanford, or the college student he was at 18, when he essentially taught himself using books at the University of Bonn’s library.

“The basic MOOC is a great thing for the top 5 percent of the student body, but not a great thing for the bottom 95 percent,” Dr. Thrun told me after he decided to shift the focus of his company last fall to concentrate on corporate and vocational training, and to charge a fee for courses.

Nearly all MOOCs originate from the world’s top universities. Their instructors are accustomed to teaching the brightest students, and may not understand the motivations, academic difficulties and self-discipline of the average student.

What’s more, for many instructors, the courses are on-the-job training in online education. Two-thirds of MOOC professors surveyed by The Chronicle of Higher Education in 2013 said they had never taught a fully online course before their first open online class. Many universities rushed their courses online in response to the initial hype, often choosing popular professors or volunteers rather than looking for the best teachers for the MOOC environment. A student who already has an advanced degree is far better equipped to navigate such a course, especially one covering difficult material.

The best online professor in a subject may teach at the local college down the street. “Could we be passing over a good instructor at a small college? Sure,” says Daphne Koller, one of the founders of Coursera. “We have limited bandwidth to partner with individual professors.”

Coursera and edX, the two main MOOC providers, are essentially acting as gatekeepers for American higher education online, replicating in their virtual world the pecking order in the physical world as determined by U.S. News & World Report rankings. Top colleges have earned their place in the rankings mostly because they admit the smartest students, spend more money on academics than anyone else and bring in the most research dollars. None of those attributes matter to the students the two providers want to attract but aren’t reaching.

Many educators take the view that, using the metrics by which we judge traditional higher education (prestige, completion rates), MOOCs have failed to fulfill their original promise.

But those metrics don’t take into account how MOOCs are being used right now. Students can register, with no financial risk, for as many courses as they want. Some might want to sample a particular lecture, or prepare a business plan for investors, or take a lesson for a presentation the next day.

Call it “just-in-time education.” These students hadn’t planned to complete the course, and they have nothing to lose when they stop taking it. The MOOC provides learning in chunks, at a student’s own pace.

Take Leo Cochrane, who already has a bachelor’s degree but took a free online class from the University of Virginia’s Darden School of Business to help expand his start-up air-purifying business. The course was perfect for the time-pressed entrepreneur. He had little inclination or money to follow a path that would take him to a traditional campus or even to an old-fashioned online course, with its rigid deadlines for lectures and completing assignments.

With a MOOC, he could watch video lectures on his iPhone while running on a treadmill and pick and choose what he needed to learn from the syllabus. MOOCs put students in control. Students can do as much or as little as they want at any time, one reason that many never complete the courses. Roughly one in 10 finishes.

The companies that rode to fame on the MOOC wave had visions (and still do) of offering unfettered elite education to the masses and driving down college tuition. But the sweet spot for MOOCs is far less inspirational and compelling. The courses have become an important supplement to classroom learning and a tool for professional development.

They are instruments in what George Siemens, who co-taught the first MOOC, in Canada in 2008, calls “the shadow learning economy,” which happens alongside formal education, much in the way textbooks supplement courses.

That is the success story for massive open online courses as they graduate from the hype cycle’s “trough of disillusionment” into the “slope of enlightenment,” on their way to the “plateau of productivity.”