We already have a tag which covers the overwhelming majority of questions of this type:
Questions about personal theories are rarely on topic here. Since many people see the existence of a tag for a given subject as strong evidence that the subject is on-topic on the site, adding a tag for them would send a completely wrong message about their topicality. To say it explicitly: it would be actively harmful to do so.
As for your comments:
I'd suggest your approach amounts to sticking our heads in the sand and pretending they don't exist.
It isn't. It amounts to saying as clearly as we can that we do not want those questions here.
Also, questions don't always get closed simply because they get lost in the flood of new question and no one does a final vote.
That last bit is completely incorrect. "No one does a final vote" isn't a reason why questions slip through the cracks - that's not how the Close Votes review queue works. If a question has pending close-votes on it, it will go to the closure review queue and it won't leave the queue until either it is closed or it gets three explicit Leave Open votes.
It can indeed happen, though, that a given off-topic question will get lost in the flood of new questions and no one does the initial vote which would kick-start the review process, and said off-topic question will remain open for months or even years. That's not a reason to give up, though - those examples are the minority and they should be fixed as and when they turn up. They're absolutely not a reason to bend site policy into harmful shapes just because our moderation is imperfect.
More importantly, though: your proposed tag is a meta tag. Meta tags are, in essence, tags that describe what a question is like, rather than what a question is about. During its initial years, Stack Overflow had a long process of dealing with meta tags, and the core lesson learned was that it is very hard to make them work appropriately and that they are very rarely worth the trouble.
This means that there isn't quite a complete blanket ban on meta tags throughout SE, but there is a very strong directive to only use them if they're really, truly needed. On this site, the only two meta tags are homework-and-exercises and resource-recommendations, both of which cover a substantial number of questions and provide a meaningful tool for the mechanics of the site moderation, and both of which are under reasonably strict rules on their usage. Your tag proposal fails that first criterion - there simply isn't the quorum of on-topic questions in that category needed to justify the trouble of a meta tag.