The topics are only used for categorizing the posts by tagging them. What you are referring to can be achieved through groups where its the relationship between members and the posts. For example a group for HelpDesk where only Q&A is enabled and a group for Ideation and Feedback where quick post is enabled.
We are making changes to the post box where users can directly choose where they are posting the content and tag the topics easier. Do you think in that case you the post box should still be removed from the main feed?