In my opinion, what we should focus on are not the questions but the answers. Even a question that asks for opinions can be answered in an objective manner. And even a question that asks for facts, can (and has been) answered by an opinion.
I generally prefer to be more generous about which questions we allow. Let us see what answers they generate, before we prematurely decide to close them. There are many questions here that are immediately voted to close, but which nevertheless inspire another user to write an illuminating answer. So let us not be hasty, but rather ask for clarification or suggest an edit. The rules are meant to help us make the Q&A helpful, not to blindly delete great Q&A just because they do not adhere to the rules.
The rules of this site are like crutches: if you can walk without them, then that was the goal all along. Forcing someone to use crutches who has healed and can walk without them contradicts the purpose of the crutches.
Also, the rules were taken from and imposed on us from a site with a very different kind of content and don't perfectly fit writing. Writing is not programming. Code works or it does not work, and it works or doesn't for everyone. Writing is different in that every kind of writing works for someone. There is no writing that no one appreciates, even if it is only the recipient of a fifth grader's orthographically challenged love letter. And even if it is only the writer him- or herself. Not everyone wants to write a bestseller.
Writing is not an objective science and there will always be a degree of subjectiveness in all questions and answers. But we can attempt to objectify this subjectivity. We can point it out, become aware of it, and enable us to consciously consider it in our emotionally charged and idiosyncratic endeavour.
It is a bit different for me to apply everything you mention in your question to all kinds of questions on Writer.SE. There are questions about the process of writing; there are questions about the content; there are questions about publishing; there are questions about formatting; and so on.
Most questions that receive an SF&F answer are SF&F questions. If we use examples from SF&F to non-SF&F questions to illustrate our point, we do so because everyone knows the Lord of the Rings, even if they haven't read or seen it, but not everyone knows Ulysses. For most question this bias in the answers does not matter, because on a level below actual content much of genre fiction works the same, and what we say about plotting fantasy can easily be applied to plotting romance or crime fiction.
I do try to take into account in my answers that some askers or users may be writing literary fiction, which is a different beast entirely. I'm not an expert in literary fiction (or film or poetry, for that matter), but if the question does not specify what the asker writes, I do try to address the possibility that they write something different than I do. But generally I think it is the askers task to formulate their question clearly and present all the necessary facts to receive a meaningful (to them) answer. It is impossible to address all possible contexts of an imprecise question and write a many-page-long essay just to not focus on SF&F. Questions that clearly ask about a specific genre, always receive answers relevant to that genre. Whether or not we have the relevant expertise or just guess based on our knowledge of another genre, is not a problem in the Q&A, but in the userbase (which largely consists of programmers with a strong affinity for SF&F).