The only ones that will take the DEN-HOU flight on F9 will most likely be the Frontier/Midwest/Republic frequent flyers. I don't see WN worrying too much about what Frontier tries.
Now on the other hand, Southwest will own the entire C gates at Denver in September and is pushing hard to increase the Originating traffic out of there. Initial results are looking very positive. I'd say Frontier has more to worry about in Denver than Southwest does in Houston.
Load out of Denver yesterday were unbelieveable, nothing less than 125 out of 137 all day long. Most flights completely full. Its all about the yield and no one usually shows their hand on that one.