In almost all cases we can tell the difference between an external link and an internal link. External links are only valid if they start with a protocol (e.g. "http://") and we automatically switch the user to external if they enter a protocol.

There is one edge case which is when the internal title begins with a protocol. Such pages exist on en.wiki, for example, but they are all redirects: https://en.wikipedia.org/wiki/Special:AllPages?from=http%3A%2F%2F&to=&namespace=0

I don't think it would be a great loss if we lost the ability to edit such links, and the simplification to the UI would we worth it.