The fact that you think "leave it alone" is an example of lacking critical thinking skills shows just how subjective this is.
On the assumption that not many people being interviewed by Microsoft in the 90s had an iPhone style plan or a desire to revolutionize cell phones, "leave it alone" is the most realistic, objectively useful answer a candidate might give. Sure, it demonstrates zero creativity, but it's totally unfair to say that person lacks critical thinking skills as their answer is the most rational.
To pass that question, you need to know the interviewer is looking for creativity and be able to make something up on the spot. That's obvious to many of us, but depending on the role I wouldn't fault someone for not being in tune to that question or ready for it.