I don't think there's a danger of South East London rail services being poor relations, they already are when you consider the frequency of nearby Southeastern services to those on the ELL extension through the Lewisham borough.
Sydenham 12tph (including 4tph on Southern), Lower Sydenham 4tph, reduces to 2tph during evenings and Sundays, also Penge East which is in walking distance.
Honor Oak Park 12tph (including Southern), Crofton Park 2tph all day.
Clearly the Sydenham line had spare paths and still has 2tph spare for additional services to Crystal Palace, yet a TfL Southeastern concession could have raised the evening service to something along the lines of what NLL/WLL passengers experience during the evenings.
Equally the DfT could with the current southeastern. Money talks!
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It's Kent County Council who frightened off the potential Dartford concession.
^^^ Railway Myth #2014