I think this is really the crux of the rejection: is the new syntax being proposed primarily to support typing, or Python in general?  Does it help both, or is one use case the motivating factor, and the other is just piggybacking on the syntactic proposal?  Quoting from the rejection email:

> The strongest argument for the new syntax comes from the typing side of
> Python. The Steering Council is not particularly convinced it is of
> significant benefit to the static type checking language, but even if it
> were, at this point we’re reluctant to add general Python syntax that only
> (or mostly) benefits the static typing language. If the syntax would be of
> great benefit to static typing, it might be time to discuss letting go of
> the requirement that the typing language be a subset of Python -- but
> whether this feature is important enough to consider that is up to the
> typing community.

The SC didn’t find general Python functionality compelling enough, or outweighing the costs.

It’s possible that the proposed syntax is really useful for typing, and less compelling for Python users in general.  And that’s totally fine, but then we’ll need to discuss whether the typing language and the general Python syntax needs to continue to track.

Cheers,
-Barry

> On Mar 15, 2021, at 13:07, Guido van Rossum <guido@python.org> wrote:
>
> Let me clarify what the typing-sig folks wanted out of this PEP. We only care about adding support for `x[*y]` (including things like `x[a, *b, c]`). We'll just update PEP 646 to add that explicitly there and hope that PEP 646 fares better than PEP 637.
>
> To fans of PEP 637 I would call out that the main reason for rejection seems to be this paragraph:
>
> The benefits of the new syntax as outlined in the PEP are not particularly strong, and community support for the new syntax seems low. The new syntax doesn’t provide an obvious way to do something that is currently error-prone, and doesn’t open up new possibilities that were not possible before. While there are certainly cases that could use the new syntax, for many of them it’s not clear that it would be a win, or that third-party libraries would indeed use the syntax. The Steering Council isn’t really convinced by any of the suggested uses in the PEP.
>
> This seems to imply that in order for a proposal like this to fare better in the future, the authors would need to line up support from specific, important communities like the scientific, data science or machine learning communities. Currently such support seems absent except for one specific package (xarray).
>
> --
> --Guido van Rossum (python.org/~guido)
> Pronouns: he/him (why is my pronoun here?)
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