This is a good point. The primary objection as I remember it to the consistent metadata proposal was that some projects might want to ship wheels to PyPI that would have different dependencies from the sdist. The reason for wanting to do that is precisely to unbundle libraries like BLAS which has all the benefits I mentioned above. Having variants and allowing them to have additional dependencies provides a way to support that case while making the metadata differences understandable for tools that want to assume consistent metadata.
The unbundling issue has been a problem for a while but looms large at the moment with free-threading threatening to double the size of a PyPI release by doubling the number of wheels that bundle identical binaries of BLAS, Flint etc.