There comes a point where they can neither get new recruits (far-fetched perhaps?), nor does anyone from Mainline wish to fly their a/c....for obvious reasons. (I presume initial recruits will be "OS frozen" for a certain period?)
Its actually possible that being on the MSL will save BA money as the young fos will join with the promise of jam tomorrow on a mainline fleet, and there'll always be some guys in BA who want an early command esp with a european base. As a separate entity it will have to compete fully with the competition.