I have quite a few issues with the proposal as present in Masem's sandbox, some of which are at the heart of the matter, as I judge from their appearance in the nutshell. Some may have been brought up above, apologies if I'm repeating someone else's argument.

For a good comparison, let us compare the current nutshell to the new, proposed, one.

Current: Topics within a fictional universe are notable if they have received substantial coverage in reliable secondary sources. Non-notable information should be deleted only when other options have been exhausted.

Proposed: Articles, including sub-articles, dealing with a work of fiction should demonstrate real-world notability from reliable sources. Such articles should be structured around the demonstrated notability for the topic, with an appropriate balance of in-universe information. Articles on fiction that fail to demonstrate notability should be improved to demonstrate notability, trimmed and/or merged into a larger article, or moved to a GFDL-compatible wiki.

"Demonstrating notability" versus "being notable"
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The first big difference is a switch from "only write articles about notable topics" to "articles need to demonstrate notability". This is quite a fundamental thing to change, and it goes against important principles such as "Deletion is not a substitute for editing" and "Don't demolish the house while it's being built." It would invalidate all deletion arguments that come down to "the topic is notable, sources exist, but no one has come down to adding them yet," i.e WP:PROBLEM. Furthermore, it indicates a switch from a notability guideline (answering the question "what subjects are notable?") to a content guideline ("how to write articles?"). Stick to the first, please.

Structuring articles around notability
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The second difference is the addition of Such articles should be structured around the demonstrated notability for the topic. Besides not being English (there is no "the notability" in any article), it is again a content guideline instead of a notability guideline.