Following this argument, it was easy to get from Forest Hill to Whitechapel by simple getting off the train at New X and catching the ELL there. Why bother spending all this money to extend the line for a seemless service? Well, because it's a pain to get off and hang around, it becomes more convenient, and perceptions change, and people use it more.

People are less likely to want to get back from Central London at midnight if they have to hang around London Bridge to change. End of story, I'm afraid.