As it happens I spent some time at King's Cross yesterday during the evening peak. Virtually every evening peak down service left with at least some spare seats, some trains left with only a few people in those cars furthest from the barriers, and the main trainshed could be seen with some 8-car trains, as well as some 5-car (Hull Trains). There is plenty of scope for today's service to be retained and extended to 12-car as far as possible - should any further growth provision be required then Thameslink could quite happily be superimposed on top of that should no other better solution be available.
Meanwhile the Moorgate service continues to see crush loading with not much relief in prospect - a bit of extra space on the new trains in place of unused middle cabs, otherwise the only 'improvement' here will be standing space in place of seating.
So that is the picture in early 2016. Come 2019 when the Thameslink Progrqamme is scheduled to complete, there will be few if any vacant seats. Come back in 2030, (if the current service was maintained, there would be plenty standing, maybe crush loading as it is for the Moorgate trains. As for 2040, which is somewhere near mid-life for the 700s, the GN services would be as packed as the Thameslink trains were before the 377s were parachuted in to remove the 4-car 319s in the peak.
The signs are already there. Large scale developments all along all commuter routes in the south-east are at various stages of implementation. This government is so embarassed at its failure to address the desperate housing shortage over the last 5 years, that it is trampling over local planning committees and the rules governing planning so that builders ready to capitalise on peoples plight, will help cover their shame.