There could be many more trains on the "Sutton Loop" but they'd have to terminate at Blackfriar's (this was to be one of the major benefits of rebuilding that station). Residents along the loop succesfully lobbied for the through service to continue, so they can't run any more trains.
Ultimately though, the situation in South London is really really hard to solve. The network is a real jumble of 2 4 and 6 track lines, with loads of flat junctions, and mixed service; local and express, and 6 terminii. The "system" is running close to capacity, but that capacity is constrained by the poor integration and segregation of all the lines and services. It would take billions and billions to sort out, and coordination between many companies and government bodies from borough to national level. So don't expect miracles any time soon.