If Southern were given permission to scrap Metro services, it'd probably be between Norwood Junction and New Cross Gate, allowing trains to run fast along the majority of the middle line, which would leave the stations inbetween on the slow line to London Overground.
In any case, the plan is to extend the platforms on our line to run 10 car services on, so I expect Southern will continue to provide services on the Sydenham line in addition to London Overground for a few more years yet.
While the East London Line is a welcome addition, it won't stop the need for a direct train service into Central London.