I think you'll find that it is exactly that. Domestic political concerns should always rule first and foremost. You are forgetting that Australia was once used for nuclear tests by the British empire without proper consultation with the people.My own theory is we hand one over and build two or more, likely PWR-3 based evolutions.I think the UK may yet spring a surprise and offer an Astute from the RN on permanent detachment/transfer to the RAN.
There are more than enough NATO assets to deal with Putin's still moth eaten fleet. A reduction in Astute numbers would be more than offset by the practical and political benefits.
The whole build rate for SSNs in the UK is massively slowed down to keep it ticking over.
It would actually benefit designers and builders to evolve the design, learn lessons and gain more experience.
Though to be strict here, the Australians could see which way the wind was blowing decades ago and could, and should have jumped in on Astute or Virginia back then.
This whole avoidance of nuclear propulsion is a product of the triumph of domestic political beliefs over hard facts.