"Has it occurred to you that a twenty to thirty year production run is a very bad thing? From a design team sustenance and supply chain obsolescence perspective?"
well one of the problems identified by others (not me) on the thread was that its was an issue because the UK stopped design and building for a period - hence the current issues.
However I was thinking that if the UK is going to build boats for Australia we'd have a long period with guaranteed jobs which would attract more people into the industry. And of course we can go for incrementable improvement - the Japanese do this with their frigate/destroyer fleet as do the Chinese. The US has been building steadily improving Arleigh Burke's for over 30 years - the latest ones being very different from the lead ship.
The supply chain will keep building kit if they know there are more orders.