Its really easy to say 'build an old plane now as that must be cheaper' right up to the point where you realise that to do so means basically building a new airplane, and then you realise its still cheaper to replace Trident with a new SSBN than it is to go down that route.
Is there not a balance? Using modern tooling and manufacturing techniques is a no-brainer (and why should it cost more?), and you can presumably substitute modern materials when there is not much impact on development and testing.
Surely the main saving is the development costs for a brand new airframe?
So I'm not sure I agree that it's basically building a "brand new airplane"