I took more than 60 courses on Coursera and edX, like the parent commenter mostly during the completely free days. About a fifth of it was top level, about a third was "easy" courses, for example history of architecture.
I did not take a single course in my profession (I'm a CS guy doing architecture work now). Most of it was in chemistry, org. chem, bio-chem., anatomy, physiology, medicinal chemistry (drug development), neuroscience, biology/genetics, clinical study design - plus a lot of statistics practical courses (using R to do stuff, there was a very good multi-course series on Coursera).
I would not have paid anything because 1) it was just for fun, I cannot make any money with what I learned, and 2) I was earning a pittance only because I was doing did for a few years while recovering my health.
I think I can generalize the second point at least a little bit - I could see a lot of people taking such courses exactly when they are not busy in their jobs. It takes waaayyyyy too much effort to do this on the side when you have an active job and especially a family on top. Therefore I think a lot of the people who would be interested are exactly those who may not have the budget to pay.
Even more so because there are plenty of alternatives (for me my learning marathon years started with 100 hours of physiology lectures in a Youtube channel, and if I had to pay anything for access there was no way I would have ever even found any for-pay courses in the first place since the whole thing was completely accidental, one thing leading to another), and also because those "certificates" don't mean a thing.
The FOR PAY model removes all the "play" part, only people with a plan andintent come to such a place in the first place. I think free education would be far better, attracting more "accidents" like myself who never planned for any of it.
Already when I took the courses I knew they were doomed to disappear (in that form) because there would be insufficient profit. I always thought some way to fund such sites so that they can provide FREE quality education would be better instead of forcing them into the usual profit constraints. Of course 99% would be "wasted" from the point of view of the monetary-minded people (for whom my own years-long for fun learning would be completely useless too), but even they might see that even just a tiny percentage of people who do benefit and who would never found the opportunity in a for-pay edu system would be worth it, globhal-economically speaking. The price is insignificant compared to any presence-based learning (and having created multimedia learning content I certainly don't undervalue the effort for even a single course, but it's a one-time effort, and the continued presence of TAs can be achieved from within the learning community, I did that too for some of the more technical courses, free support to other learners helped me learn that much more by having to research other people's questions).