I don't doubt this is the case but seems to be a retrograde step, especially for the Tyne Valley - Durham Coast route. Forcing people to change trains means a proportion won't bother using the train at all. Not everyone is wanting to travel to Newcastle Central.
Long distance trains pick up more delays than those on simpler there and back routes. Of all the major TOCs c2Cc does best on punctuality largely because it runs there and back services into Fenchurch Street and Liverpool Street.
Contrast that with another extreme, East Midlands Liverpool-Norwich. Lots of through passengers overlapping at multiple points along the route. Great to stay aboard one train for a longer journey. Not so great for those wanting to catch a connecting service.
This might be a subject for its own thread, but those wanting a punctual service from Newcastle to Hexham or Hartlepool might do better with a train starting from Newcastle. The benefits have to be weighed of through journeys for relatively few with punctual journeys for those starting or changing in Newcastle.
If lots do want to travel across the city a good case might be made to continue as it is, however adding another cross city service for the Northumberland Line may complicate timetabling too much.