This is exactly what has happened with the expansion of the PEP into a mechanism for standardizing the metadata in an sdist. We in fact had basically this same discussion here.
I think this idea is too much in that it changes the nature of pyproject.toml in an undesirable way and it is not enough, in that it’s not going to be a very effective way of speeding anything up or improving the way tools can scrape metadata from the ecosystem. Even if we pass this as is, today, I don’t think a significant fraction of the ecosystem will have adopted it within 2 years. I imagine adoption will be particularly slow among big and fundamental projects that everyone depends on — which are likely to be more conservative with their build systems for various reasons. If we were to go with a “standardize the status quo” approach, we could upgrade huge swathes of the ecosystem overnight by implementing standardized sdists in setuptools.
Also, to circle back to the rejection reason:
To be clear, my suggestion was that PEP 621 data is canonical, when specified. The difference would be that pyproject.toml would remain purely an input file, and tools that need to access dynamic fields from pyproject.toml would still need to be resolved by a call prepare_metadata_for_build_wheel.
Is this something other backend authors agree on? I think the main benefit of something like this is that you can have a unified tutorial for how to specify a lot of what goes into your package, but I suppose it’s fair to say that since this doesn’t specify enough to actually build a package (doesn’t specify the contents), then it wouldn’t generate terribly useful tutorials (other than possibly a nice “how to specify dependencies” tutorial, which could have a decent amount of meat in it). The other benefit is that it allows people to write a standards-compliant PEP 621 → METADATA library for re-use by backend authors (though of course someone can just write one anyway and then “whatever that library does” becomes the standard).
If we’re pivoting, I say we pivot to an informational PEP intended to give backend authors a ready-made description of how to design a way to specify metadata in pyproject.toml.