There was a lot of development required for that sub. We were basically intending to do it by contracted variations. Probably the biggest flaw in that plan was the way the design contracts were structured. Imagine buying the designs for a 3 story house, and converting it into a single story factory, but by contract variation, every item that is changed is individually costed and accounted, then designed, separately. Then there were issues about sovereignty and Australian content. I wonder if it was built to fail in the contracts deliberately.
The idea of a 100m conventional submarine with 4 or 6 diesels engines, a huge modern battery, would have still been a very promising concept. The concept should have probably just been a clean sheet~6000t submarine, with an ultra low drag hull, with at least 4 large diesels, compartmented/cooled lithium phosphate batteries. Basically snorted with something like a 50% indiscretion rate at ~15kts its way to the area of operations, then putted around at say 8kts on storage, with maybe handful of snorts, then snorted back. Its not a SSN, but it would be tangibly much faster than regular subs, and much faster than collins.
Albacore set a few records for submarine speed and endurance with her silver zinc batteries.
en.wikipedia.org
Having a lot of diesels, means when you are charging, you can put huge currents into batteries if they can handle it. So with 6 diesels, you can charge 300% faster than just with two diesels, so much shorter indiscretion rates while on station, or much faster transits etc. I believe the original attack was going to have 6 diesels, this got shifted to four. This gives a much larger power budget, and transit speed. Most subs have two, Collins has 3. Barracuda is a pretty low drag submarine, the french had to give up a lot of SSN "features" to get unenriched uranium reactors working well, because you trade so much power density.
Again, I think for subs, we are better thinking about sitting around choke points in Sunda, Malacca etc. In cooler deep water. With a sub with really quality sensors, long range vertical launch strike, able to hunt shipping around the choke points. A conventional could definitely do that. Being able to fire off ~32-64 VLS of tomahawks, or drones, or UAV's or UUVs or LRASM, or NSM. But also hunt shipping. Listen, watch, be present but pretty much undetectable once it gets to the AOO.
The US has real strategic issues making more subs. the AUKUS or Virginia sub should not be a given.
IT rubs up against very expensive things like shipyards and nuclear fuel production etc.