The idea of improving user experience is an abstract construct that requires subjective interpretation, but so is the notion of moving a project from uncertainty toward certainty. Whether a state is more or less certain is subject to interpretation just as is whether a state provides a better or worse user experience than a prior state.
So why does the definition even matter? The defintion of design that we choose affects the discussion we have around the discipline of design. I believe a discussion around improving user experience better captures the power and nuance of the modern discipline of design than a discussion around moving from less certainty to more certainty.
Regarding your specific example, I would submit that the question is not whether a new design provides a better experience than a previous design, but whether the new design improves user experience in comparison to life without the designed artifact at all. A nonviable product will die and will not help anybody, so viability is a legitimate aspect of design.