This is very timely. I've been toying with the idea of creating a course ever since I started writing. A lot of people have encouraged me to and I certainly have the material. But I've come to the same conclusion as you: I just don't want to spend the time on marketing, selling, and support. I'd rather be doing the core services that I love. I don't want to get distracted by a side-projects that's likely to be less lucrative than what I'm already doing. Then I'd quickly resent the time I have to spend on it and lose motivation.

Sometimes doing one thing really well is all you need.