I agree with most of this, but I don't understand why you'd treat FKs as business logic. IMO, it's clearly a part of the data structure. Like, say if you have a domain that looks like:

    type User = {
      username: string;
      posts: Post[];
    }

    type Post = {
      content: string;
    }

(for the DDD-brained, assume User is an aggregate and Post is a value object here)

The natural way to represent this in the database is:

    CREATE TABLE users (
        username text PRIMARY KEY
    );

    CREATE TABLE posts (
        author text NOT NULL REFERENCES users (username),
        content text NOT NULL
    );

I just don't see why you'd do it in any other way.