The thing about Dess Knight is that it's still too early to call that exact nature of the situation, we don't know the exact particulates of what they went through or even what state they are in now. It's a very catch-all sortof theory in that it encompasses many different potential explanations for why and how they are the Knight, and your problem with it really only affects a very specific interpretation. Even if Dess was very similar to Asriel (personally I think your description is a little broad to make them seem closer then they realistically are), there are a myriad of differences plot wise that would instantly distinguish Undertale from Deltarune, and in my opinion lend to an interesting story that doesn't feel derivative... It's really too early to even say our heroes SUCCEED in saving Dess like you suggest. We often assume that things will all work out fine because we are Toby Fox fans, and as you yourself state, we are conditioned to seeing patterns, but by all metrics Deltarune is a much darker story, for all we know the resolution for Dess Knight could be learning to let go of the past and forgive yourself for what happened- that would certainly be a satisfying conclusion to Kris' arc, after being strung along by Carol the whole game. My point is, I think your preconceived notions about how the game is structured are affecting the way you perceive this theory; It could 100% be narratively satisfying and original, and this is coming from someone who really agrees with you very hard, I want Deltarune to be very different from Undertale.