Maybe what's losing market share is the monolithic 'CMS'. Content, for lack of a better term, has long since outgrown the hoary confines of tools like WP.
Small businesses need mostly static sites that are easy to update content on. The non-static functionality some businesses need is generally quite constrained: appointment setting and information request forms being the two most likely, and there are plenty of X-as-a-service offerings in that space.
