I get what you say about Moorgate to Bank being one stop on the Northern, just as Liverpool Street to Bank is one stop on the Central. Many other OSIs are one stop tube journeys: Waterloo to Embankment; Charing + to Embankment; Marylebone to Baker Street. Let’s see what happens!
On a similar point GN stations to TfL Rail stations via Moorgate & Liverpool street currently charges for a zone 1 tube journey between Moorgate and Liverpool Street as there is no OSI and the connection via the new Elizabeth line station is not yet open. Once this becomes an Elizabeth Line journey, I presume these fares will reduce on the basis that there is a direct connection at Moorgate; I wonder if using a Greater Anglia train (including the destinations served by the Elizabeth line) from Liverpool Street (using the Liverpool Street LU-NR OSI) will continue to include the tube journey as there would be no way of knowing if the Moorgate to Liverpool street journey had been made by tube or on foot.
I wonder if we need a separate Elizabeth line forum …