While you could promote the idea of it replacing the missile deterant, it would be more complementary so you would most likely keep some of the missiles if not all of them. Certainly a value argument could be made.
I would imagine it is entirely doable given the right set of circumstances. But the argument then goes if you could squeeze a little more then you could get a carrier big enough to operate SH/F35C off.
I still think the Dozen or two F-35B buy is the most likely (which is not likely at all). I think the biggest gain is really sig/intel/visual gathering, and you can do this with out putting squads of aircraft in the air all the time. While UAV's will fill this niche, sometimes you need people actually out there to do something and sometimes respond. Sometimes an RC plane does not send the message you want to send.
You have some strike capability, you also get some A2A capability (certainly enough for our immediate region).
It would also offer greater ability to train with aircraft in blue water operations, also training and becomming familiar with handling aircraft with allied partners.
While the LHD won't be a full carrier, you could certainly conduct training operations of her. The purchase of a light (cavour plus style) carrier would also bolster Oz's amphibious capability to actually be able to conduct the mission we want (although a 3rd LHD would be better in that argument). Ideally you would have both.
Maxium Realistic RAN hypothetical:
1x Dedicated carrier with amphibious capability with 24 -30(~18-24 on ship) F-35B
3x LHD (2 ordered, 3 ship (prob LPD) promised)
5x AWD (3 ordered, 1 vaugely mentioned)
8x Anzac II
12x Subs
20x OCV
2x AOR
Out of this you could create two groupings, an amphib or carrier group. You would have enough escorts to back this up (2 AWD for each group, with Anzacs fleshing it out with some spare) or you could even lighten up one (supposing its operating with the other group) and create an additional surface action group (1 Awd 2 Anzacs) while still supporting the two amphib or 1 amphib/1 carrier group.
While only having one carrier, the 3rd LHD could fill in for her when needed (as purely a carrier) an basically make the situation workable. Its role could be shared by any of the LHD making entirely flexable. It would only be doing this in severe crisis that could not be planed for ahead of time. While a carrier it would be a poor choice, but with 3 x LHD operational the load could be somewhat shared (between 75,000t-90,000t of ships). (combined fuel loads, deck space, hanger space, helos etc)
You could conduct a full amphibious assault entirely self supported (just). You could conduct simulantous amphibious landings, or consectuative landings and support them near indefinately. The ADF could do pretty much and thing the USN could do just on a smaller less contested scale. We could operate east and west fleets. While there would have to be some flexability in the fleet make up, there is huge capability.
The problems come when you water this plan down. 4 AWD instead of 5 means your going to be short on lynch pin escorts. You lose that 3 grouping capability (not too bad) and you can still escort everything else. say a US ship being intergrated into your group things would again be looking up, but if you stay at 3 AWD seems completely unlikely as you proberly can't even train for that level of warefare nor sustain it. 2 LHD means you really don't have enough to fall back on when your carrier is out. Less than 24 F-35B means you can't really field enough aircraft. No carrier means you can't really do anything except training.
Look at the spanish armada
1 x carrier
1 x LHD
2 x LPD
5 x Destroyers (4 here 1 on order)
6 x Frigates
4 x Subs (4 new ordered)
21 x patrol craft
2 x AOR and a fleet oiler
Looks some what simular to my hypothetical. They have 2 LPD + LHD compared to my 3 LHD, still very simular. We could make 2 X LHD + 1 LPD work, but it would be less flexable and less capable particularly with aircraft. We could do this, spain already does this a country of comparible wealth and population sitting in safe protective europe. They still have a credible missile defence as well.