The alternative is now a half baked conversion of the Bankstown line with a screwed up connection with the existing DD network at Bankstown that will be more costly to timetable than now. They can laugh all they like (if indeed you actually discussed this), but it just goes to show the lack of thinking on their side.
Half baked, indeed. Moreso, there's little mention or indication of the benefits for those west of Bankstown at this time (putting aside network benefits/those serviced directly by metro).
As a commuter from Carramar with one of the longest commutes one the network in terms of commuting time/per km from Central (particularly due to large amount of stations/stop) it seems like ensuring a relatively quick interchange (maintained out of the peak hours in particular) at Bankstown is the easiest yet most difficult due to frequency on the down end.
Peak hour travel times could remain largely similar due to a metro every 3 minutes in the peak. However, off peak is where the benefits could be achieved for the benefit of commuters west of Bankstown.
If on the metro up end, a 3 minute wait/ interchange to the CBD in kff peak and the same on the down could be achieved e.g. even with the half hour frequency on the down end from Bankstown if every 3rd metro in the off peak could manage to have a 3 minute interchange with a service to Liverpool it could deliver benefits. ( I realise this is not possible with the two off peak Metro's given half hourly DD service in off peak but may be achievable on every third service to Liverpool, and likewise with the service to Lidcombe).
However, if not, and it means largely similar travel time to the CBD with the current commute time and an interchange (e.g 10 minute wait time for a metro to the CBD in the off peak once getting off from a DD) then it's a bit of a wasted opportunity and double whammy, losing a direct service to the CBD and extended wait times at Bankstown.
Guess it's wait and see. In an ideal, and my preferred world, the stations west of Bankstown would be upgraded to Metro as well