Without even trying to figure business cases out, I think it should be obvious that going after busy aviation routes should succeed without more intracity transport: there's obviously "enough" public transport for aviation to succeed there (where that "enough" might be the fact there are plenty of car hire agencies at the airport along with one or two bus services!).
Unless the alternate mechanism is going to have the same termini,like as the air routes (as in, at the airport itself), it still likely needs additional local transit, even if the existing network is adequate to support the airport.
