Thameslink is a mainline route!
And has been since its inception in 1988. Bedford-Brighton was the backbone of Thameslink in 1988 and still is now.
Why is it essential for Hertfordshire to have a direct service to East Croydon but not - say - Essex or Buckinghamshire? The only difference is of course that people will to some extent have adapted their travel patterns because of Thameslink.
The key point is that, starting in 1988, the long distance traffic generated the passengers and the revenue. In the first year after Thameslink opened there was 11% growth on the Bedford line.
I'd also note that the attempt to serve lots of mainline destinations also leaves stations between Kentish Town and Mill Hill Broadway with a (by London standards) rather poor service due to so many trains running non-stop through them, but I'm sure infrastructure issues and lack of capacity on the MML contributes to that.
The Canal Tunnel onto the GN is there to provide enough north of the river destinations for 24 tph, the MML on its own doesn't have the capacity for that. Putting the long distance GN trains through the core also has the advantage of releasing platform capacity at Kings Cross for more ECML trains.
the GN’s inner-suburban service has tended to be a bit unusual in that it doesn’t actually carry vast numbers of people. I’d say this is for two reasons:
(1) that much of the inner-suburban service has been hived off to LU (High Barnet, Edgware) or has stiff competition from nearby LU lines which are able to offer a far more frequent service (Northern and Piccadilly lines). The GN main line out to Welwyn is very much affected by this, the Hertford line slightly less so.
(2) The decision to run GN inner-suburban trains into Moorgate whilst convenient for commuters has essentially fixed the train formations at 120 metres, which is comparatively short for metro operations.
That's a good summary. I'd add that, out as far as Winchmore Hill, the Piccadilly line is an alternative to the Hertford loop, and both the Enfield area and Hertford have Liverpool Street alternatives.
running the whole GN suburban network as either King’s Cross or Thameslink core services, with a minimum train length of 8 cars. To achieve this would of course need some platform extensions in a few places
Platform extensions in lots of places!
My solution would not be to extend all of the platforms on the Hertford loop. Instead rebuild Gordon Hill as an 8 car station with two terminal platforms in the middle and the through lines on the outside. Then have 2tph Welwyn-Sevenoaks and 2tph Gordon Hill-Orpington which would give 4tph Alexandra Palace-Bromley South. Moorgate would keep 2tph to Welwyn and 2tph to Stevenage via Hertford, so Welwyn and Gordon Hill-Finsbury Park would also be 4tph.
The bit I haven't worked out is where to put the Peterborough trains, if they no longer go through the core!
And thats because only about half the drivers (and thus trains they drive) are trained on ATO. When they are, 120 second headways will be much more common. As it happens my train hime yesterday arrived at St P 60 seconds after the previous service had departed (wheels start rolling). Had my train not been early, we could easily have left in under a minute of dwell, ie less than 120 seconds headway. (Yes, both services were in ATO).
ATO does make a difference, but it makes least difference when and where it matters most. St Pancras going north I'd say was the easiest station in the core to achieve a 120 seconds interval, because every train is coming in from a relatively long run from Farringdon, with most running in without coming to a stand, and it has spacious platforms. It is more difficult to achieve a 60 seconds reoccupation time if the following train is coming in off a red light outside the station. I have watched this going northbound at Blackfriars, with its mix of trains coming from either London Bridge or Elephant. And less than 60 seconds of dwell is much more difficult to achieve at Farringdon or City Thameslink than at St Pancras. Most Thameslink users know about Farringdon, but City Thameslink has surprisingly narrow, and hence crowded, platforms, for a modern station.