Thanks for the thoughtful feedback!
I agree that OP was missing a stronger motivation (or really any motivation that wasn’t just a single pointer to PEP 751). I edited it to include more concrete/nuanced motivation(s).
I’ll spoil it here: A big one is remote distribution. Copying one file is easy. Copying two is oddly more than twice as hard.
I’m wary of inline-script metadata to becoming too verbose
I’ll call a spade a spade, it definitely will come extremely verbose. Instead of arguing that, I’d rather discuss: “Is the metadata being verbose harmful enough that it wouldn’t be worth the addition?” I.e. “The pros don’t outweigh the cons” or “I’d rather workaround the challenges of not having the locked dependency information in the script itself than deal with them in the metadata”.