One aspect of online learning

emphasized in 'Binge Learning'.... "This seems like a more natural way to learn than traditional educational structures can offer: develop an interest and mercilessly indulge it until another interest supersedes it. It is a method that conserves the mental energy associated with willpower, leaving more of the brain’s resources to focus on the material itself. Since it relies on the student actually being interested in the class, it is hard to fit into a physical schooling environment, where classes have to begin on a schedule, go slow enough for everyone to keep up, and run in parallel with other classes.

Online education also saves the resources associated with context switching. Humans are notoriously bad multitaskers. Each time a high school student has to change classes, she has to quickly stifle the thoughts and questions raised in previous classes to focus on the current class. She has to expend mental resources remembering where the previous session of the current class left off. And when she returns to the class that stimulated the thoughts that had to be stifled, she may not recall them. Far better to focus on—or even to binge on—one subject until she is at a good stopping point." 

I am not sure about all the points above. If one does not practice and use what one learns, it can disappear quickly. The traditional method of learning over a long period several things also involves sleeping over what one is learning and helps retaining it. With older students with specific interests and motivation, I am sure online learning helps. I am sure that online learning is here to stay but it is still evolving and people have to figure out what works for them for which topics in which combinations.