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.
Quite, but since the 1960s this same area has seen a huge amount of growth and development, New Towns at Milton Keynes population ~270,000, Northampton was made a New Town and more than doubled in size, to population ~230,000 today, Peterborough was made a New Town now ~205,000. On a smaller scale, Corby was made a new town, growing from ~20,000 to ~60,000 today.
Yet the rail network, in terms of the connectivity, has barely improved since the 1960s, if anything it got worse with the closure of the Market Harborough to Northampton line. You've got Leicester and Nottingham straight up the M1, but there's no direct trains from Northampton and MK, and the indirect route means journey times are a joke. Between Peterborough and Northampton there's Wellingborough, Kettering and Corby all effectively stranded on the MML and cut-off from the major towns at each end.
In terms of supporting regional transport, people working and commuting between regional towns and cities, the rail network in this area is not fit for purpose.