While educators do require an incentive, this is not necessarily the case for just experts in the field. There are numerous sites I can go to and converse directly with people who have PhDs or other education in the relevant subject matter. Of course, just because they are knowledgeable with the subject does not imply that they will be good at explaining it, so there is definitely a trade off. I think it would be accurate to say that as the difficulty of the subject increases, the need for a educator at hand increases. It is easy to teach yourself, say, calculus online, but more advanced maths such as topology require a teacher. The question now becomes- are these more advanced topics harder to teach yourself due to difficulty or due to lack of online resources?