Suppose we discover or invent or whatever the final Ultimate theory. But even then could it offer us an answer to the most basic question: Why there is something rather than nothing?
I believe that no theory can offer such an explanation. This is because a theory (Y) is based on a set of observations or axioms (X).
So, if X => Y, we would still need to explain X. Then we can say W => X. See, it is an infinite regress.
The only superfluous solution which the Ultimate theory can hope to provide is something like Y => W. So it makes a circular theory which does no better explaining the universe.