The notion of "topic" or "subject" (of a Stack Exchange site, or anything in general) is completely arbitrary. The broader concept is called categorization.
Enforcing categories on the natural world is a human construct that is completely synthesized, meaning that human beings imagine categories where they don't actually exist. There is not, out in the world, any particular object or group of objects that you could objectively call a category.
Since categories are arbitrary, there is no objectively "right" or "wrong" approach to categorization. Similarly, there is no objectively "right" or "wrong" approach to determining which Stack Exchange sites should exist, and what the topicality rules of those sites should be.
Now, there are two completely separate perspectives from which to talk about Super User's topicality rules:
- Descriptive: What are the current topicality rules that are established as the community consensus right now? -- The answer to this question is, for the most part, purely factual, objective, and not based on opinion whatsoever.
- Prescriptive: What should be the topicality rules in the future, as compared to what they are now? Any prescriptive argument for what topicality rules should be in the future needs to include a persuasive component arguing why a given proposal is superior to the current model.
Let's look at how your question could be interpreted, both descriptively and prescriptively.
One thing that is objectively true is the answer to the question, "Right this moment, what are the currently-accepted topics or categories that are on-topic for Super User?" You can find the answer to this question here.
Within the topicality rules as currently defined, Excel questions are certainly on-topic, from what I can determine. Is your argument that Excel questions are off-topic based on the current topicality rules of Super User? If so, you need to provide some evidence for this claim, such as interpreting the topicality rules to describe why you think this is the case.
Other than claiming that you believe (against the current accepted consensus of Super User) that Excel questions are off-topic, you have provided no evidence whatsoever for why we should adopt what you say is the correct categorization.
What persuasive reasoning do you have to present to us to suggest that the current topicality rules could be improved by segregating out Excel questions into a different Stack Exchange network website and declaring them off-topic for Super User?
Since your question does not provide any reasoning whatsoever along those lines, I have downvoted your question.