Simple, it doesn't depend on requiring the content to have lots of identifiers. While you may convince some web developers to just add more identifiers, you can't convince everyone. And for largely unstructured plain text like mailing list archives, there's not really a logical way to do that anyways.

Re: making people upset. I mean, nearly anything will upset people. One way a search engine could incentivize people to add more usable fragment identifiers is by ranking pages with them higher, but I am certain many wouldn't be thrilled by that.

If this is implemented I don't think it will actually raise serious compatibility issues, and it's not a huge deal to remove it later, it should degrade softly.