I have to agree largely with @slugster's answer. Like slugster says, there are very many ways to do things with all the subtleties and variances across all styles.
Is the current state with subjective questions desirable and I am just
barking up the wrong tree?
If the current set of subjective questions don't fit the guidelines in the Good Subjective, Bad Subjective blog post (most is good, all would be ideal), then they should be closed and have an explanation for why it is closed. There should be some learning aspect to the subjective questions that is beneficial to the community here. I think that might be the value you are seeking in determining whether to keep a subjective question or not. Correspondingly, answers should also be written to cover those points from the blog post. Hopefully, a question is asked such that an answer does meet those criteria. I think we have a mix of both good subjective ones and some mediocre ones.
If no one is learning anything, if people are just posting opinions, then the question probably wasn't very good to begin with. It should be closed with the hope that it can be improved.
Assuming that it isn't desirable, what can we do to emphasize the
Back-It-Up principle or some equivalent?
I almost want to say years (hours, actually) of training could be a useful marker, though that does not necessarily invite deeper understanding. Someone who trains in various martial arts may have a wider breadth of knowledge but may not have as much a depth of understanding one particular art and vice versa. If people do answer something that is out of their experience but provide something valuable, then they should also understand and acknowledge their shortcomings and be respectful of whatever boundaries they are treading.
Stating qualifications doesn't have to explicit; that should be conveyed in what someone writes in an answer about personal experiences. I would also place more value on answers that do an exceptional job of explaining and communicating knowledge to an audience, but can do so in a way that does meet the six subjective guidelines.