It is not so much that HTML5 can replace Flash by matching it feature by feature. It is more that the HTML5 stack (including JavaScript and CSS) can be used to build the same sort of rich web applications for which Flash was the only viable option before. Because HTML holds the promise to also address some of the shortcoming of the Flash platform, there is a potential that it can replace Flash in this space in the future.
This discussion is very much centered around the technology that is used to render the application at the users end. It does not address the ease with which authors are actually able to create the content which will be delivered to their users. You already mentioned that Flash is associated with a set of commercial tools which are specifically built and marketed by Adobe to address the need of rich-content authors. Such solutions for HTML5 are not that much adopted yet. For developers who view authoring mostly as coding this is not a big concern and they will not have a hard time getting traction with HTML5 (and many already do). For those authors that don't have a developer/coding background, this may be another story and for them the viability of HTML5 will greatly depend upon the availability of integrated authoring tools. Maybe the Adobe Flash tools will output to HTML5 instead of SWF in a few years...
