The ECML has never been and is still not just for running long trains between the big cities that sit directly on it. It is a complex mix of direct services, through services to or from significant off route destinations, connecting and crossing local services, commuting services, together with several key freight flows, each meeting individual commercial and social objectives.
The value here is in the total mix and whether that particular service meets the objective set for it. A path is not a wasted if it is a 5 car or 9 car - the only waste is if no one is travelling on the train, whatever length it is. Running long emptyish trains just for the heck of it is the real waste of money.
I commuted on the GN for 25 years. In that time I always regarded train paths through the Welwyn 2 track section as the critical resource. In that time it was always the case that, when the Welwyn 2 track section was required to operate at full capacity, all trains were full length 8-12 cars, with the objective that those trains would be full, except for the 2tph that called at Welwyn North. That was the way to get the maximum number of people over the Welwyn Viaduct at peak times.
In my commuting days that was only for 2 short periods covering 0730-0930 arrivals at Kings Cross and 1700-1900 departures on Mondays to Fridays. Other traffic, such as open access operators and freight, only got paths at other times. What has changed is that there is now peak demand for train paths through the Welwyn 2 track section that extends all the way from 0730 to 1900.
In the longer term, how practical would it be for the open access operators only able to run 5-car trains to couple them together between London and Doncaster/York/Newcastle?
In those days GN, and its predecessors, needed to run short 4 car trains to serve stations north of Royston. But it still only ran 8 car trains through the Welwyn 2 track section at peak times, by doing splitting/joining at Letchworth, Royston and Cambridge. This is the approach that should now be adopted with the open access operators. For example, in today's timetable, the 1023/1213 Lumo departures from Kings Cross could run 10 cars to Newcastle and split, with the rear units then joining at Newcastle onto the 1356/1550 departures from Edinburgh. The GN services to/from Kings Lynn operated like this for years with splitting and joining at Cambridge. Suboptimal use of the Welwyn 2 track section should not result from operational constraints hundreds of miles away.