Yes hiding the collapse button would work, but I just checked and there is one more way users may switch back to drawer mode. If you press the - key when you are in the forum, we open the chat drawer:
And in this function we call chatStateManager.prefersDrawer(), which sets the local storage preference. We have had some more discussions lately internally about allowing a chat-only mode of Discourse, including an experiment in this direction, as well as ideas around drawer mode.
Not sure when the time will come for us to focus on this more, @mcwumbly may have some input here or @lindsey .