> simply because it's a third party dependency

Not simply. This is one one of the most important reasons NOT to propagate something through your code. How many millions codebases use it is irrelevant.

>How many millions codebases use it is irrelevant.

It is relevant, because it speaks to the reliability of the dependency. `pydantic` has 24.7k Github stars and was last updated 52 minutes ago.

Adding a random dependency `dacite`, which has 1.9k Github stars, no one has ever heard of, and was last updated 4 months ago, introduces way more complexity and sources of instabilities than propagating `pydantic`.