Could an evolved Hobart class (the proposed design) fill an Air warfare destroyers role to the same capability?
Could the Global Combat Ship for fill the role of an Air Warfare Destroyer?
Could the proposed FREMM frigate fill the role of an Air Warfare Destroyer?
Finally which would be the best fit for the role?
The question of which one of the three Future Frigate contenders could act as a 'substitute or back up' for the AWD's, and which one would be the 'best fit' to fill that role, is pretty much a moot point.
As it appears, regardless which one of the three contenders chosen, they will likely have the same CEAFAR/CEAMOUNT systems, and possibly the same overall sensor package, regardless of which hull design chosen.
A better question would probably be to ask what will that sensor package deliver for the selected ship design?
As it appears at the moment CEAFAR (on the Anzac class) works fine with the shorter range ESSM weapon, but as ADMk2 mentioned, CEAFAR is 'scaleable', CEA on their website refer to it as being "scalable in size and power to meet a broad range of applications, suitable from 'Corvettes to Cruisers' ".
And as ADMk2 also said, it appears that CEAFAR is developing the capability to support SM-2 and SM-6.
So that really is the answer to the question, what level of capability will CEAFAR and the other sensors provide to the Future Frigates?
If it is no more capable that installed in the Anzac class, then ESSM is probably where it's at, but if it includes that SM-2/-6 capability, then they will have a capability 'closer' to the AWD's.
So that is one half of the question, the other half is how does the Government intend to 'equip' the ships when it comes to weapons load and mix? That I wouldn't have a clue.
Bottom line will be, 'if' the sensors are there to support (and get the best out of), either SM-2/-6, then yes a more longer range AAW capability is certainly possible for the Future Frigates.
And again, if not, the Future Frigates AAW capability will probably not be much better than the ESSM equipped Anzacs.
As it stands at the moment, the RAN will have two distinct types of large surface combatants, both with a different primary focus, the AWD's with a primary AAW capability (but also an ASuW and ASW capability), the Future Frigates, a primary ASW capability (but also an ASuW capability and an AAW capability of sorts, what that is who knows??).