S, I understand the desire for an easier "compromise" solution to the transition problem. But I think the system that you're suggesting would create a jumble of functionality that would be just as bad, if not worse, for the users who are trying to navigate it.
If people are going to work together on wiki talk pages, they need to be able to talk to each other. Seeing two different communication systems operating on different pages would be confusing enough. Having to choose between the two systems on the same page is just impossible for users to figure out. It tries to please everyone and would actually suit no one.
Right now, there are some important gaps in the use cases that Flow supports. We have to figure out how to create multi-editable Collaboration space, and we have to figure out short-term and long-term solutions for what to do with the yellow Metadata boxes. These are hard problems, and it's going to take us a while to figure them out.
But I think saying "let's do both at once" is basically giving up on the idea that it's possible to make something that's better than blank wiki talk pages and the template-heavy cultural practices that we currently see on the big WPs. We can do better than just add another confusing element to the page.