I use a route with 1/2hr interval during the day and 20min at peak. It is surprising how relaxing it is to realise that there will be another in 20min as you watch the just missed bus disappear, compared with the drag of waiting 1/2hr. If you can't plan precisely around the bus timetable (eg appointments 5min before scheduled arrival, random task completion) it is very encouraging to opt for public transport if it is basically turn up, no more than a short wait, and go and if delayed there is a later service to use. Thus the point is, not what present usage is but what usage can be developed over time. (Melbourne trams are a good example of how users automatically opt for the trams when services are frequent. Very often, on week days at least, if you miss a tram the next is already in sight.)
Commenting on the airport aspect, it is easy to overlook the substantial non-aviation commercial development that exists there. A bigger concern however, is that a large number of flights depart very early in the morning. Taking into account check-in times, the present bus service is not useful for substantial numbers of passengers, especially from beyond the City. This is a major failing of public transport in general. Each service is treated in isolation rather than being considered as a whole co-ordinated system from origin to destination.