To be clear, I don’t really care all that much about this—the lack of the setting and the reliance on the workarounds simply feel at odds with nearly everything else I’ve experienced with Discourse, which is otherwise delightfully customizable.
Plus, having one message composer be constrained with a hard-coded limit and another unconstrained feels less efficient than having a single abstracted composer. Not that I know anything about how the code works under the hood, of course, and code that’s written is always more efficient than code that’s not written. 
That would also address the issue, and in a more discoverable way than a setting.