Why?
It will require more stock (currently the London - Leeds service tend to have an half-hour turnaround at Leeds so not enough time for the little jollies for what will mostly be local passengers), all will require reversals at Leeds [unless you decide to ignore Wakefield and send the services via Hambleton).
Similar to the Newcastle terminators being extended to Sunderland, it will require bi-modes and also pathing might be an issue so the 800 has to reach Pelaw junction before a metro service, then clear out of the way at Sunderland due to platform space, for what will probably be mainly used by local passengers wanting to cut down the journey time vice the metro.
Easiest way for the East Coast is to keep it simple, maybe even getting rid of those "once a day" oddities.
I think you make a number of good points.
Whilst there's always going to be crayon-wielding enthusiasts desperate for more "quirky" services (in a functional world of clock face timetables and simple routes), and some civic desire to put our backwater town "on the map" (by giving it a London link), there's a lot of wibble on this thread!
The shortest train on the ECML (a 130m IET) is still going to be overkill at places like Scarborough/ Grimsby, a complete waste of capacity. Even more so if you are talking a 260m IET (assuming the infrastructure would allow it).
You'd be importing delays from single line areas and other bottlenecks (e.g. pathing problems with the T&W Metro) onto services passing over Welwyn.
You'd potentially struggle for paths on lines like Leeds - Skipton (fine early in the morning/ late at night, but where are the paths for an irregular service through Keighley... and why aren't Northern using them for EMU services?).
Seems a lot of trouble to go to for the sake of variety.
Yes that is perhaps a point worth hammering home (just as it was yesterday in front of the Public Accounts Committee). The InterCity East Coast farnchise is profitable. I think the figure mentioned by the Michael Griffiths of Stagecoach was £200m profit for last year. The problem isn't that the franchise doesn't make money. It's that DfT's premium payments swallow all the profit up (and then some).
No, no, no, you're ruining the simplistic narrative that East Coast were amazing because "they made a profit" (as if that makes them unique on the ECML)!
Population sizes do not determine rail demand and usage. Of course it's a factor, but not the deciding one. Hasn't this point been made so many times?
Certain places are big but have low demand - such as Bradford and Sunderland. Nobody wants to go, and the locals don't leave so much. Whereas other places, either for leisure, business, historical railway (railhead or junction) or other reasons, punch well above their population weight... and I mean O/D rather than connections - York, Chester, Newcastle, Bath, Reading, Cambridge...
Good points - it's not as simple as looking at the population figure on Wikipedia.
There are some large places in the UK that don't attract significant numbers of outsiders travelling there other than for football games - there are some smaller places that get large numbers of tourists/ students all year round.
As a geographer, I can't let this claim pass.
It's bigger but in a way which is entirely meaningless in any sensible way. Newcastle is ~300,000; Bradford, 528,000. But Bradford incorporates Keighley, Shipley, Bingley, Ilkley, etc etc etc; Newcastle is a fragment of the Tyneside conurbation. Newcastle, North Tyneside + Gateshead is at around 750,000, and this doesn't include several parts of the conurbation.
Certainly, Leeds-Bradford is bigger than Tyne-Wear but the idea that the claim that 'Bradford is bigger than Newcastle' is of any use to planning a rail network is a stupid one and reveals either idiotic pedantry, or mindless jingoism.
I might just copy and paste this every time someone tries to claim that Bradford is one of the largest cities in the UK!
(these "my town is bigger than yours" arguments are always skewed - some places like Bradford/ Sheffield have very loose geographical boundaries including Ilkley/ Stocksbridge, some places like Manchester/ Nottingham have tightly drawn boundaries that don't include a lot of places that outsiders would identify as being part of that city - e.g. Beeston, Salford).