Do we really want to change the button from something else
to destructive once text has been entered? What do we do about users whose browsers don't support JavaScript?
We need to be careful with setting too strict of rules in situations like this. I find the color of the button is important for many users to distinguish the difference between actions at first glance. In the case of Flow, where we have Cancel (or Discard -- the wording of which actually makes more sense), Preview, and Reply/Add topic, I don't want to have two buttons side-by-side of the exact same color that perform drastically different actions (Preview and Cancel).