Kulga "The sidebar is not a critical piece of flarum and having it as a extension would probably suit it better. It's been there since as long as I've seen flarum though - so it's probably one of the few non-critical parts of the core."
I see no link betwin those two things. Just because it's there since the beginning, doesen't means anything but the fact that it was there at the beginning. It doesen't means that it should stay there, or that is good to have it there, or, for that fact, that it's a non-critical part of the core.
Sure, getting Flarul out without Composer might be more important, shortly followed by adding Themes support, if it doesen't already haves, but it doesen't, in any way, mean that since it's been there first it should stay there for now, or forever. This is how evolution works: You adapt for the enviroment.
What about having an option to have the sidebar to the left or to the right (and it would only imply loading another CSS file to change the position of the sidebar) or not shown (a CSS file to hide the sidebar and make the rest have the width:100%).
Sidenote: The comment window might really use a bar with the buttons to apply markdown to the selected part of the comment. But a extension would do...