Yeah, I think you may be right. Just playing with ideas here. 
I will say that third column is great for me on a 4k 32" screen, but is not an option for many smaller screens, much less mobile. So the UI of it still needs some fancy thinking, I suspect.
But my main concern at present is having much clearer indications of where there is chat available and making it easy to view it without necessarily “joining”, etc. Right now it’s pretty darn clunky, you have to choose to “browse”, then choose to “preview” a channel or “join”, etc. Compared to Discord and similar systems it is very not-smooth. 
It sounds like Chat in Topic is going to be removed as a feature anyway, judging from recent posts from Sam, so maybe much of this consideration is no longer necessary. I think it actually makes some sense to remove chat-on-topic for now, focus on category-based chat, and then if there seems to be need for/value in per-topic chat, it can be added in sensibly later, based on existing, longer experience with the overall chat functions and how they’re used by various communities. I definitely feel quite scattered with the chat experience at present, and category-based will provide some needed focus.