That's exactly my point.

How long has that service been established? Granted there will have been minor tweaks and changes, but not the wholesale changes seen on other services or routes.
It doesn't need garish advertising or funky branding. People know the service, know where they can go, how much it will cost and dont need to worry about zones or missing an infrequent connection.
The 39/40 covers a massive proportion of the city and the suburbs, is popular and is recognisable as a Stagecoach service.

Whether it is the 'flagship' service is open to debate - but it hits all of the criteria according to the definition mentioned earlier.