Future of Northern City Line

Ah, thanks for that, Rob!

Yes, City Thameslink is indeed a long way from Moorgate and if FCC withdrew its services on the NCL, there would be a lot more people changing from FCC at St Pancras Low Level to the Northern line or Circle/H&C/Met at Kings Cross St Pancras in order to reach the Moorgate area or there would be people changing at Finsbury Park onto the Muswell line.

Also, you're probably right in saying that the Thameslink lines through St P, City Thameslink and Blackfriars couldn't accommodate the GNE services as well as the current Thameslink services and the Peterborough and Cambridge/King's Lynn ones.

As for the NCL, it is eerily quiet during weekday off-peak hours although the service is more frequent than the District line Olympia shuttle. 6tph (every 10 mins) as opposed to 4tph (every 15 mins).

Finally, I have read that Haringey Council wouldn't welcome the rebuilding of the FP-Highgate and Highgate-Ally Pally lines as that would get rid of the Parkland Walk, an amenity that Haringey is very proud of.

On the other hand, there has been a campaign (The Muswell Hill Metro Group) to reinstate the Highgate-Ally Pally line going for years....

If they could take the Parkland Walk back it would be much like the Croydon Tramlink as it would use abandoned railway land. There would be a few areas where this would be difficult.

The point was that people could take a train into the city from Muswell Hill, but it could be a solution.

The line has not been built on.
I like the idea of a junction onto the GOBLIN, but there is quite a difference in the height between the two lines. The GOBLIN has a road above it and then the Parkland walk is above that again.
Where it would terminate would be quite simple in my opinion. via South Tottenham and Lea Bridge to Stratford or Liverpool St. Or even Lea Bridge and then westbound onto the North london Line!

The problem with Finsbury Park is getting across the ECML

The plan is for fast services (Peterborough and King's Lynn) along the Northern City Line to take the Thameslink route, while the slow services, which are the ones running to Moorgate (barring weekend/late evening slow services to King's Cross and fast peak services to Moorgate).
The planned service pattern, according to Always Touch Out, is:

Bedford - London Bridge - Brighton (4tph)

Bedford - London Bridge - Dartford (4tph)
via Sidcup (2tph)
via Bexley (2tph)

Luton - Elephant & Castle - Sutton, Wimbledon (4tph)
clockwise via Sutton then Wimbledon (2tph)
anticlockwise via Wimbledon then Sutton (2tph)

St Albans - Elephant & Castle - Sevenoaks (2tph)

Peterborough - London Bridge - Horsham / East Grinstead (4tph)
Horsham, inc Gatwick Airport (2tph)
East Grinstead (2tph)

King's Lynn - London Bridge - Sevenoaks - Ashford (2tph)

Cambridge - London Bridge - Eastbourne / Littlehampton (2tph)
Eastbourne (1tph)
Littlehampton (1tph)

Letchworth - London Bridge - Guildford (2tph)

Ah, sorry, I thought that the lines it connects with north of Finsbury Park are also considered to be the Northern City Line. It does not change the rest of the post, though, at is does mean that fast services should be going via Thameslink while the slow services keep going onto the Northern City. I really can't imagine them closing the Northern City.

I thought they were getting new trains for the 2000 service?

Finsbury Park has a large sign saying "Great Northern Electrics"

Presumably the 319s released by Thameslink can go round to replace earlier Mk3 based units such as the 313-317s

That's partly because the A stock and C stock are perfectly suited for what they do and were built to do a rather specialised job.

That and the fact that A stock isn't obsolete as Underground train design hasn't evolved a huge amount since then. Slam door stock on NR was obsolete when they were withdrawn but A stock is still good at what it does.

Yeah, there are no imminent plans to scrap anything that survived the slam-door kill off. There's talk of replacing various non-313 PEP stocks (314/315/507/8 - Glasgow, East London and Liverpool commuter stock), but that'll take a while and may not lead to their scrapping. Even the Pacers seem safe in the short term. The 317s certainly are.

The oldest trains are the 313s and HSTs built ca. 1976, and they aren't going anywhere for a good decade. A lot of them will hit the 40 year mark easily.