Expanded production capacity simply must be the solution. The question is how this is to be achieved. No doubt the answer is complicated and has occupied much of the negotiations to date. But whatever solution is arrived at, it will certainly take some years to bring to fruition, which indeed suggests that Australia operating SSNs is more a prospect for the 2040s than the 2030s.
The criticism that Australia's notional SSNs will simply take too long to arrive is the only one that has found any real purchase with establishment figures to date. Peter Dutton, the former Defence Minister under the previous Liberal government (and current leader of the Liberal opposition), under whom AUKUS and the nuclear submarine announcement was orchestrated, claims that these schedule concerns are overblown. Specifically he claims that Washington would've been willing to transfer two Virginia-class submarines from the production line
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So when you look at statements from relevant American figures that push back against this notion of simply adding more orders to the existing American production line, which effectively means taking orders from USN and giving them to Australia, the implication is that Dutton is full of shit, and therefore the schedule concerns are very real, and therefore the wisdom of the whole enterprise is in question. Hence the prospect (hope!) that the forthcoming defence review may recommend a further conventional submarine acquisition program as a "Plan B".
In any case we will know a lot more in a few months.