That's not true! There were many closures north of London that reduced connectivity, for example:
Princes Risborough-Thame-Oxford
Bletchley-Oxford
Welwyn-Luton-Leighton Buzzard
Bedford-Northampton
Bedford-Hitchin
Bedford-Sandy-Cambridge
Cambridge-St Ives-Huntingdon-Kettering
Peterborough-Wellingborough-Northampton
Peterborough-Market Harborough-Rugby
Bishops Stortford-Braintree
Cambridge-Haverhill-Sudbury
In particular, the ECML/MML are across the road from each other in London, but, going north, the first rail route linking the two is Peterborough-Leicester. Between London and Peterborough/Leicester six routes going east/west between the ECML and MML were closed.
I'm glad you've made that list - saves me doing (something like) it. I am not sure of the GE lines, but you have missed out the lines through Northampton - Towcester - Banbury (I'm not exactly sure of the route, TBH). There was also Higham Ferrers to Wellingborough, though that barely counts.
But you well illustrate the point. If you consider the Midland Main line today, compared to what there was in 1960, it is almost bereft of any E-W connnections for 100 miles between London and Leicester. All you have is one measly slow line from Bedford to Bletchley - hardly (at least until E-W rail gets built) a 'strategic' connection.
But the real problem here (and on the WCML - nothing much between Watford and Coventry) and even more on the ECML (nothing between Hitchen and Peterborough) is that in 1960 you had a string of medium-sized towns (eg Luton, Bedford, Wellinboro, Kettering) and small towns (eg Mkt Harboro) which justified and needed connections to London, but then what?
All of those connections you mention connect towns, none of which had universities or major attractions except Oxford and Cambridge. They were mostly short branch lines with some sort of low commuter flows (eg Northampton - Bedford) or longer country routes with even vaguer traffic potential. How many people needed to travel between Kettering and Huntingdon regularly, either then or even now? And the service, just three trains each way per day, reflected this.
Arguably, there were only two 'strategic routes' Oxford - Bletchley - Bedford - Cambridge and Northampton - Wellingboro - Peterborough, and not even these could survive. Once the lines going north hit proper cities, a route did survive: Brum - Leicester - Peterborough - Cambridge/Norwich
And then (to keep on topic), amazingly things improve! You have Crewe - Derby (though but a shadow of its former self) and Derby - Nottingham - Grantham - Skeggy and Nottingham - Newark - Lincoln with a few going on to Grimsby.
North of that second horizontal line, you have all manner of E-W routes once you hit Sheffield.
So, I would argue, that shows how much better off the Midlands and north are in terms of general connections to multiple points.
It didn't help, of course, that one city - city - city route (I'm talking about Nottingham - Leicester - Coventry) was recreated and then destroyed by the WCML 'upgrade' of the 2000s.