PAK:

As I said, your only argument is that since we don't know for sure what will happen, anything could happen. Although it's true, it's entirely worthless in a rational debate, because it means ANYTHING can be considered a valid theory (eg. Irvine = Ultimecia etc.). For a rational debate to work in a meaningful way, a theory can only be considered valid if it meets certain criteria. It needs to have substantial, preferably unambiguous backing from the game itself (eg. through dialogue or misc. information etc.), must be reasonable when put in context with the entire story (ie. if the theory goes entirely against the entire rest of the story, it probably isn't that good), and must contain as few unfounded assumptions as possible. If faced with several competing theories, we then use the principle of parsimony to determine the better theory.

R=U not only suffers from lack of evidence, but it is unreasonable when placed in context with the rest of the story, and is MUCH less parsiminous than other theories on Ultimecia's background. Thus although R=U is technically speaking as possible as Irvine = Ultimecia or whatever, it cannot be said to be a valid theory. At least not in the context of rational debate.