Some interesting points throughout these conversations. 

Dan mentioned growth due to vehicle allocations. I'm sure there are examples of this happening, just as there will be examples of numbers stagnating or not growing.
The 25 was hourly in its previous incarnation as the 725 (allocated the same vehicles as the 723, 231 etc) . It changed to the X25 at some point and recieved the Loylnes (still in their pink colour scheme from their days on the 21) and a brand name tied to the Bobby Robson charity.
Numbers clearly didn't grow to the levels anticipated (marketing and promotion done on the cheap perhaps?), the X prefix was dropped and it continued, with a mixed allocation of vehicles - often older Vxxx ERG Solars, having the same stopping arrangements (as far as I'm aware), as it did as the X25.
Trips were reduced and if there was a decker allocated, it was generally because of interworking with scholars.

Within that same period, the new housing development in Birtley was progressing. An ideal opportunity to grow and develop the route, pushing it to a greater frequency and offering it as an alternative to the Durham Road services.
Except, nothing happened. It was kept as a second tier service.

Whether punters on Portobello Road or Portmeads look on the 21 with envy, whether they don't bother with the 25 due to it using older vehicles or because of its frequency - only they can tell the powers that be.
As it can be quicker boarding one of the regular 21 or X21 services and walking up or hoping to land on an 82, maybe there are some clues there. 
Ditto, for those living in Eighton Banks and the plethora of services to Wrekenton, before a short walk for the final leg. 

There have already been points made about the people of Barley Mow and Vigo not using the bus to the QE. Timings, frequency and cost will just be some of the reasons they travel to/from the hospital in car or taxi and not bus. Im yet to see or hear of any positive pro-active or reactive measures to any drop in numbers on the 25 or other services. Just the traditional older vehicle allocation, lower frequencies or removing of the connections passengers had. 

There's many a reason these services are less popular and a half hearted push in the past, isn't suddenly going to fix the service or attract additional punters. Needless to say the lack of pushing since the Bobby Robson branded Loylnes were carted off elsewhere, won't have been any help. 

I posted elsewhere following on from Rob44's post about the 25 stopping at the other side of Gateshead Interchange to the 21. Yet there's nothing that I can see or that I'm aware of, that lets punters know of that alternative. 

Oh and to keep it relevant to the initial subject.
Those L94's should have gone a good while back imo and if the eminox upgrade programme had been managed differently, maybe they would have been. 
The allocation of L94's on the 194 when new, was what re-ignited my interest in public transport again. Despite working in transport at the time, it wasnt something which floated my boat on a personal level. The L94's changed that. 
However, I'm not sure their durability from a passenger perspective sustained itself as long as they were designed to.