Based on my experience with Wikipedia in French, I have some difficulties to identify how Flow would replace the subpages in the article talk space.
In Flow Portal/Functional Specifications/Boards and Topics#Board, I read "Each MediaWiki page object may have one (and only one) Board associated with it.". What exactly is considered a MediaWiki page? How could we use Flow to replace subpages of talk pages (for which there is no corresponding subpage of the main page)? On WP:FR, we use this a lot for evaludation purposes, such as in the process towards good articles and featured content.
On that same page, I also read "If a Board's associated Page is deleted, the Board itself is deleted.". Well... One case where we use subpages of the article talk namespace on WP:FR is for the Articles for Deletion process. And we definitely need those discussions to survive the possible actual deletion of the article!