I can see the argument for that. However I can see the argument for having message packages where they are most used or produced, as well. Another thing to consider is having one single package that is depended on by everything else. This can potentially get annoying when that single package changes. (I can’t remember and ROS2 doesn’t compile on my iPad so I can’t check, but I recall that all messages in a package get compiled into a single library.) So even if they all go in a separate repository then we still may want to put them in separate packages.
Also, remember that having things in a repository does not change how much binary you put on a target. That’s determined by what you actually compile and use.
If rviz is going to display the message, it needs access to the message.