Unfortunately Southern Rail are organisationally separate from London Overground and London Transport so that may impact commom sense things like running longer trains when the Overground is out. I suspect that Southern Rail make not a lot out of their inner London routes.

I would really like to see Southern Rail inner London routes (and those of other rail companies) to be migrated to Sadiq Khan/London Transport. I am not saying that would fix the problem - but at least it would be under one management.