I think the more important chunk of his argument was the downstream dependency; you now aren't testing your interfaces against all expected usage in the code yo

I think the more important chunk of his argument was the downstream dependency; you now aren't testing your interfaces against all expected usage in the code you're writing, you're now building it against the expected usage of any and all arbitrary code now and in the future. As well as, as soon as you unseal it, then you have to assume it's being used somewhere, and you're suddenly dealing with a much more exposed and expansive API to maintain.