Six million people a year would use train to Melbourne Airport

[dajop]

Maybe the train line should go MEL to Arden which continues direct to central CBD / Flinders, much more central that southernX.

That has been mentioned as a (the?) pathway into the CBD for the Airport train. Either way, a change at Sunshine or Footscray would allow access to the metro tunnel if the airport train is destined for loop/SX or vice versa.

Apparently there's a Sunshine Station Skybus, which achieves the same thing

Yes true. Although an Arden stop would/should be quicker for those not on the Sunbury, Geelong or Ballarat lines.