I would view that Adelaide already has its hands full with the Hunter and sub programs. I would be concerned that additional builds would distract them from what is already a difficult requirement. And they have their own cost pressures.
Adelaide would I believe need more facilities as the existing ones are sized for the existing activities. So an upgrade would be required regardless. Why Austal? Well a lot of the infrastructure and skill base is already there. I suspect it is a cheaper option than building a new yard and team from scratch. Is it a perfect solution? Not even close, but it is what we have, and beggars can't be choosers.
I would also proposition that the naval architects and other engineering disciplines located in Perth with Austal have no desire to move interstate. If Austal folded or another yard opened on the east coast, the majority would seek other employment locally.
For what its worth, Melbourne's Williamstown shipyard would have been the perfect solution, but this is no longer available.
Added to that I don't think BAE is well known for sharing with other competitors. I suspect BAE working alongside Mitsubishi or Hanwa for instance would lead to problems (aka fist fight).
In the broader picture, I don't see this is about cost. A second ship production facility provides future options as well as the capacity to do the current workload. If the world gets worse, then we have more levers to pull.
I don't see it as much different from the desire to have a missile factory in Australia, or an AI drone plant with Boeing, or for that matter a boxer/redback/hunter production line. None of them are the cheap option. All of them are about self sufficiency (and possibly electoral votes).
The sub facilities are to be in it's area of Osborne and I believe it will also have it's own workforce, so that people assigned to work on surface vessels will not also be tasked to work on the subs, as some of the required skillsets are I believe slightly different.
As I understand the surface shipbuilding activities at ASC, I believe there is space two have two hulls under construction at the same time, and that the original
Hunter-class build plan had the 2nd hull getting started before the lead ship was completed and in the water. Now that the order number has been reduced by a third, this means that the ASC yard will run out of surface vessel work several years earlier than expected, unless/until gov't places more/new orders with them.
My take on the whole build a warship in WA thing is that as a matter or reality, one is talking about establishing a new yard and workforce in WA to carry out such work, regardless of which company might end up getting awarded the contract. I believe this because none of the facilities in WA currently have built proper warships and similarly, neither have the existing workforces. Now I readily admit I am uncertain just how much input the Austal engineering staff in WA might have had in the Austal USA build of the
Independence-class LCS as opposed to engineering staff with General Dynamics, I would certainly not count any of that experience as something worth using towards building steel mono-hulled warships. TBH given how poorly the LCS programme has turned out, I would probably be inclined to exclude that experience as either being irrelevant, or evidence that what/how the design processes were ill-used.
One of the things which I am concerned about is the longer-term implications of Australia building up a 2nd yard for warship production and workforce. Specifically what happens when the order which the yard was established for is completed. Is Australia going to be able to guarantee sufficient orders get placed in a timely fashion so that both yards are able to keep going? Given past history, I suspect the answer would be, "no."