(19 Dec 2020, 11:08 am)Andreos1 wrote Some interesting thoughts/ideas there.
Whilst I think reinstating or extending services could be an option, I do think the wider network needs looking at for some of them to work. 

As an example, I was aware of an organisation set up a few years back which was often mentioned on here. 
The Fencehouses Popular Front (FPF), were an action group set up by disgruntled residents of Fencehouses. There were all sorts of schemes planned and plotted to disrupt services across the region. 
Residents had been left with the poor 71 service, which only offered limited options compared to the 21A, 160, 163, 177 and 178 which it 'replaced'. 
Similarly, the X94 and X3/X4 offered a lot more than the then the M1 which took its place. 
Whilst the 4 and its increase in frequency is appreciated, it still doesn't allow residents of a wide area direct routes to Newcastle or Gateshead and therefore both journey times and fares are a lot greater than they once were. 

Reinstating a service from/via Fencehouses to Newcastle or Gateshead would conflict with the 21 or X1, depending which route it took. Even though it would be popular and could see the FPF cease its attacks. 

A while back, I brought probability in to the forum and questioned whether or not a shared corridor and the ratio of services along the route had any impact on the success or longevity of some routes. The two corridors I brought up at the time were Newcastle Road and Ryhope/Stockton Road. I predicted the likes of the infrequent X3 and 238 would go and the more frequent services would last. 
Using Eldon Square - Harlow Green as an example: Per hour there are - 6x 21, 2x X21, 2x 25 and 2x X12.
Ratio/probability will be 6:2:2:2. If you're waiting for the first bus to rock up, chances are it will be the 21. The other buses are an option if there's no 21 at the stand. But the probability of that happening is very low. 
Stick in a twice hourly 21A in to the mix and there's still more of chance of boarding a 21 to Harlow Green than any of the other individual services. 

That's forgetting any marketing/route branding and brand or operator preference.

If you look at your 721 example, you're potentially going to see the same pattern on Old Durham Road and Durham Road. 
Whilst there will be punters needing to get to/from the QE from various parts Birtley, I don't think that will be enough to sustain the service.
Between Chester and Birtley the ratio is 6:2:2:1 (21, X21, X12 and X22). Add the 721 in to the picture and you've got 6:2:2:1:1. The 21 is in all probability going to sweep the passengers up travelling between the two points with both options.
It will be a similar pattern between Newcastle/Gateshead and the QE or the QE and Wrekenton where the frequent X1 and 56 is the dominant force and you have the other local services such as the 57 and 58 dropping in and out of the mix.

Surely the better option would be to just extend some of the short 21's instead, there's 6 of them you could do something with rather than sticking another service on top of them. (There's 8 21's btw not 6 (every 7.5 mins)).

For the North Tyneside Hospital from S. Valley restoring the 57 to North Shields would be the easiest option and the QE to Birtley restoring the 25 along there. The bottom half already has an hourly service btw with the 28 it's Portobello etc where the complaints are coming from.