I don't think Dess will be a perfect victim. Asriel wasn't one in Undertale, and as Undertale's parallel story, I believe Deltarune will do the same with Dess. Her transformation into the Roaring Knight may have been involuntary, but I think there's an element of wish fulfillment for her in it as well; freedom from expectations, from mistakes, from who she used to be. But Chapter 4 told us that you can never wash it all away.
The Roaring Knight doesn't want to be Dess again, it's afraid of being Dess again, of being trapped by that perception. I think it can move on anyways. This whole game is about fighting stagnation, breaking out of the roles imposed on you. If Dess finds freedom from herself in the Prophecy, then to save her, we need to show her how to find freedom outside of it.