Comment by p_l

IPv6 explicitly supports all sorts of transitional technology, including being able to map v4 addresses to v6 that are used with translation gateways connecting from v6 to v4 (widely used in mobile networks to provide any v4 access).

That still requires that if you have used BSD Sockets before getaddrinfo was added (or like many, didn't learn about it for years) then you had to rewrite the parts of your application that are responsible for handling connections.

So the very thing you're advocating for exists