If you look at the development area, it is a triangular site with Vauxhall (Victoria line) in one corner, Battersea Park (east London line) on another and Wandsworth town (also East London line) at the third. Queen's town road is also next to the site, serving Waterloo. This is an incredibly well connected area. Diverting this branch of the northern line to provide two stations and no future potential for further expansion, is not sensible. It's entirely different to the jubilee line extension which served not just a new development but provided many existing areas with new connections / regeneration.