Our customers expect that they can be as productive in their Apps programs as they can be sitting at the UI. When they can not, this is surprising and disappointing. Historically, G Suite has not insisted on this kind of completeness. However, we want to move towards that goal, hence this requirement for a plan, while we only advise completeness itself.
For major changes, however, the desire for completeness will be amplified.
Each action that can be performed in the UI should be possible to do via a public API. For example, if there is a button in the UI to add a widget, there should be some way to add a widget using the API. This simplifies the user’s understanding of the API, and makes it easier to record UI actions to be replayed via the API.
The question of when the completeness ought to be achieved is not fixed. Each team must have a plan for API completeness, even if it is a simple statement, such as, “When we are given resources to do so.” (We would prefer more, but if that is your plan, then so be it.)