With all the talk of Chapter 5 supposedly being the last "fun adventure" Chapter that's supposed to be a break from the "dark clouds" of the Prophecy, and the implication that it's gonna be similar in first three, whether or not the game is gonna subvert that, I think it makes the most sense for Rude Buster to come back as the main theme for one last time in order to emphasize that tone.  Like if the Chapter does get really dark and you just hear "From Now On" when fighting these cute harmless enemies like those Floradinns that kind spoils the true tone of the Chapter, no?  

With Chapter 5 looking to be the last "normal" Chapter and a farewell I think it's the best spot to conclude this battle theme.  Minibosses like presumably the Flower Pot guy could even give us more uses of "Ruder Buster".

  
I think "From Now On" will likely be the battle theme for Chapter 6 and mayyybe 7 if it doesn't have its own special battle theme.

Some might say that "From Now On" is a confirmation that this will be the main battle theme "from now on", but that'd be like saying Rude Buster can only play in fights with the Rude Buster- when Rude Buster plays in many fights where you can't use the Rude Buster or even Susie, particularly the Noelle enemy fights, and many fights in Chapter 1.

I think "Rude Buster" might be the main battle theme of Chapter 5.