The not deserved comment was tongue in cheek and sarcasm doesn't work in text format, Ok I get that now, and yesterday had been a long day.

I still prefer the railhead theory of transport as advocated by some transports analysts at privatisation. i.e. timetables would be run for good connections, so trains from Chathill, Hexham, Sunderland would all arrive into Newcastle in enough time to allow a good connection onto the ECML service (XC at the time having a less regular service than now).
This was part of the reasoning given when the Metro was extended - it would allow better connection from Sunderland (from South Hylton round through City Centre and up the coast) straight into Newcastle Central.
If the railhead theory had been adopted, services into Darlington from Middlesborough would have been similarly timed.
You mention lots of these head offices, but last time i was through Sunderland - 18 months ago, they seemed to be building such offices nearer to the A19, and Doxford Park, itself not far from the A690 (which leads to Durham).
Given that, like most City centre's, Sunderland has congestion and no 'quick' route by car to the station, and given the longer journey time from Sunderland as opposed to Durham, combined with the location of the major business parks on the Durham side of Sunderland with a direct road link to Durham, and the faster journey times, I'm not sure how many people will be willing to sacrifice choice in travel time and shorter journey times, just to travel from Sunderland.
Journey times from Doxford to Durham can be almost as low as to Sunderland given Durham station's location on the edge of town near the ring road.
Also, with NXEC, Business customers can have a range of departures through the evening (ticket permitting), however with GC this is limited, so I see their market as being more aimed at leisure travel to London, and not Business.
Finally, GC has said, and analysts have estimated that the York stop was the stop required to make money, so the Business customers most likely to choose GC, and those who will make or break GC's business will be York commuters and Business people, rather than Sunderland, or Cleveland. IMO
PS> And maybe I should have said this after my last post, all this is opinion.