I don't know the area around East Durham much at all, but I'd think anything they're running into Sunderland would perhaps suffer from the same problem as the X2 down Durham Road.When I've used the X2 many of the fare paying passengers have day/weekly tickets and either boarded the X1 or has come off another service like I've seen people jump on in Newcastle and Chester-Le-Street and then get on the 7 or other services like the 43. Like you say Arriva don't have anything after Framwellgate Moor apart from the 42 from Brasside and Arnsion Centre although I doubt many have connected from these two services.
We know the passenger flow on Durham Road is thick and fast, and also that there's a massive gulf between what you'd see in the figures for the 21 and for the X2. Problem with the X2 though, is that between Pity Me and Eldon Square, there is not a single other Arriva connection in sight. Some 50 minutes of travel time, with god knows how many stops.
If there were say a few local services in Chester, a few in Gateshead, and the odd connection to Washington? Probably a completely different story.
Saying about Arriva running services in Chester-Le-Street they did have the 726 from Langley Park to Chester which ran hourly Monday to Saturdays between school runs and it only lasted a few months before being axed because the passenger numbers were pretty much non existent.