I dunno, it just seems a bit cynical to say that there has to be a financial incentive, rather than just social benefit. DLR/Jubilee/Northern line to Battersea all had/have an element of private funding, so naturally the financials were the main driver. I guess the new London Overground routes - although admittedly far cheaper than a tube extension - would be an example of a new scheme designed to improve the overall infrastructure, without an immediately obvious economic driver. Saying that, I suppose any scheme which strengthens the transport network and allows future scalability is bound to have long term financial benefits.