I'm sure that, as in the past, the X20 change is mostly a case of allocating the Durham - Langley Park section of the route to the different service(s) changing the resources required, rather than increasing or decreasing them. It's previously been the standalone 14 and and part of the 20A.
The X5/15 is currently half of what it was 6 years ago so maybe this is an attempt to make it more viable or to move resource demand away from Deptford.
And, after all, it's a constant that retail parks haven't been well served by bus services and one of the things that GNE has done right, routing the 21 via the Arnison centre, has proved that there is a demand for it.
RE: Go North East - State of the Fleet