My hypothesis is technical writing is different from non-technical writing and those who have mastered both the skills are in huge demand and have better career path than being a coach.
Probably also true. I'm sure I make more in my current role than I would coaching people, unless I was able to either leverage that into some sort of weird social media/Youtube phenom, or scale it up by hiring other people (and then you just end up having the professional-services-firm problem).