I thought the reason they couldn't run more services at 2tph was due to pathing issues with all the London Midland trains. I'm sure a member of Southern staff said that to me once when on a WLL train.
That is not really news though, it is exactly what was said in the London and SE RUS of 2011 in the section discussing the WLL. The aim was explicitly for 2 tph peak service (Option I1) of 8 car trains (Option I2) from such time as the WCML timetable was re-cast, and the stock became available from Thameslink; hence a quadrupling of capacity on the WLL.
However the decision on the second service's destination was to be made later. (This latter question being only whether or not it would run to Milton Keynes or not - possibly only Watford Junction.)
The thing is that the WCML timetable IS supposedly being re-cast in the timescales of this GTR franchise (there are all sorts of WCML open access decisions waiting for it) and possibly well before 2018. So the 'timetabling argument' should have been solvable in the run up to 2018, yet as we have seen the GTR route brief no longer mentions it.
I guess it's possible that the analysis explaining why the proposal has disappeared will be included when the forthcoming Sussex route strategy (that replaces RUSs) is published.