If I had a £1 for every time I've read a "all flights are loaded, except for XYZ, it must be getting dropped" post, only to read a "oh, it's on sale now" reply a few days later, then I'd have enough to buy a pint in an airside bar.
While I have no experience in airline planning, I'd imagine the sensible thing to do would be to load all the regular flights first, then sit down and look at the remaining flights, and plan out aircraft availability. They will be waiting on a few departments to reach decisions - do they plan on retiring any aircraft? What new deliveries do we have coming? Are there any other routes needing an upgrade/downgrade?
Once the regular flights are sorted, they will know how much capacity they have available, and can start analysing the performance of Newcastle, and how well they expect it to do next year.
When a route is launched, United will have signed 1 year contracts with all the ground and catering companies. They will need to speak to these companies and negotiate a new price for next year, and decide whether they want to use an alternative handler. They'll also need to speak to the council, and other stakeholders, to see what they can offer in terms of marketing assistance.
They will also need to look at the alternatives - if Newcastle doesn't return, where do you re-deploy the aircraft? Is it worth sticking it out and hoping demand improves, or risk launching something new, with all the upfront costs that involves? What kind of deals are they offering?
Only once all of this has been done can a decision be made on whether the route is going to be feasible.
This will all take time. So the fact EDI is on sale, and NCL isn't is meaningless.