We'll have to wait until the new SLL launches, yet I remember when the ELL launched here that everyone would still use London Bridge and passenger habits changed thanks to the easier connections to the tube along the route.

Clapham High Street to Canary Wharf will be a much easier journey with a quick change to the Jubilee line compared to the current Northern/Jubilee via London Bridge, which is also more expensive as it requires Zone 1.

The important thing is TfL promote the new line (like they did here) properly to encourage passengers in Clapham and Wandsworth Road to switch from the Northern to the Overground for City and Canary Wharf journeys which will take some of the strain from the tube.

In any case, because of platform extension works at Battersea Park, the current SLL would be axed anyway.