Biggest problem for the rumours is that there is no chance of any more PWR-2 reactors. The order for the core for the last, for Agincourt, was ordered in 2012 and the line is now shut. The production plant is now in the middle of a major rebuild to produce the PWR-3 cores for Dreadnaught and SSNR - which is too big for the Astute hull.
if there is a question it is whether the UK could manage with a fleet of 5 Astute and a crew/use sharing arrangement with Australia for the last 2 under SSNR arrives.
The RN only had 5 Swiftsure and went up to 7 for Trafalgar. If tasking for patrols in the southern hemisphere is shared with Australia under AUKUS, would 5 surface for Atlantic operations supporting Vanguard/Dreadnaught patrols and other duties?