What makes you think that the possibility of obtaining the extra two airframes (to take the fleet to 10) is not still on the table?
I think so far, that everything has gone to plan, to the 'script' announced previously, to repeat what I said above:
*October 2014 - Then Def Min Johnston said that at least two extra C-17A's would be acquired (7 and 8) and that the Government was looking into an additional two (9 and 10), but that final decision on the 'additional' two (9 and 10) would be made as part of the Force Structure Review for the new DWP (which seemed to clearly indicate that an announcement on 9 and 10 would not be until the new DWP).
*November 2014 - The Government obtains US Congressional approval for 'up to four' C-17A's, yes of course just because something is approved doesn't necessarily mean it will actually happen, all true, but still the approval for the four is there.
* April 2015 - Today the PM and the new Def Min announce the purchase of airframes 7 and 8 and the base upgrade too.
If you think I'm reading it wrong, tell me? But as I said, it appears to all be going to plan, I really wasn't expecting to see an announcement on airframes 9 and 10 at this stage, I'm sure when the new DWP is announced we will all find out then if it's going to happen or not, that is unless before hand there is some clear announcement that those two extra are not being proceeded with or that Boeing announces that the rest of the 'white tails' have all be sold and the RAAF doesn't appear in that announcement.
Cheers,