The point I was trying to make was that with this file:
some-diagnostic:
en: "..."
fr: "..."
It's easy to see that if I want to change this message and I only speak English, I need to create a new diagnostic ID, whereas if the translations are in separate files and I'm in the en file and all I see is this:
some-diagnostic -> "..."
I have to look in every other translation file to see if the text is safe to update without renaming the ID.
I think @Chris_Lattner3's suggestion to see how other projects handle this is a good one, some may have developed some kind of linter or automated tooling to enforce best practices in this area. Even with the combined format and no additional tooling, it would be possible to make mistakes, just less likely.
Edit: It could also just become best practice to rename an ID any time the text changes, even if the string isn't translated yet. That seems tricky to enforce though