While GA has a minimal online presence, I'd agree that there is a notable lack of courses designed to take a practicing professional from intermediate to advanced. This is likely due to the problem of finding a classroom sized audience at any given time that shares a significant enough background on a topic yet still finding that there common set of things they all lack.

While I have ideas on how this could be solved to some extent, most of the time it's going to be a limitation of finding an audience that's willing to pay the same amount as someone who's looking to start a new career. Some of these students have a very strong drive to start something new. An established practitioner is unlikely to have feel as strongly and thus much less likely to pay the same sorts of rates.