So we would today be stuck with the same level of service as in the sixties?
No, of course not But as I mentioned elsewhere, any rationalisation of the Midland (or any other line for that matter) had it happened* would make future enhancements to services more difficult. This is a hypothetical thread.
(* Of course, to some extent, it did happen: lots of four track was made to two, or three, with total closure north of Melton and singling Manton - Glendon.)
The joint line was several miles longer than the Aylesbury line and would have had to share tracks with Paddington expresses and locals. When will people understand that the GC "London" extension never reached London on its own, was only double track and relied on using the Metropolitan line or the longer diversion via High Wyccombe. In contrast, the Midland had four tracks from London to north of Kettering (and then alternative routes to the north).
One equally might retort: "When will people understand" that mention of a four-track main line to a BRB member c 1960 would immediately elicit the response "Does traffic justify FOUR tracks?" - and before anyone could answer - "Can't we cut that to two? Or one? Or none?" ?
So, the joint line was a bit longer than the Midland? Do you really think that if there were a decent case for closing the near 45 route miles- of which about 30 was four track - between Bedford and Wigston in the 60s that the BRB would worry about running an extra 6-7 miles extra on the GC-GWR joint for Leicester/Sheffield trains? Of course they wouldn't. Once WCML electrification had been done, the line north of Bedford would have been closed toute de suite. But luckily, there was no such case.
Once again, this is a hypothetical thread. I would not personaly advocate keeping the GC versus the Midland. It was a no contest. Rather, I see the situation rather more like:
OK, we'll close the GC - that's a cert.
Now, can we do to downsize or close the Midland north of Bedford?
In the end, due to the existance of Wellingborugh, Kettering and Harborough - and the cost of diverting other trains to Nuneaton - the answer to the second question was: Not too much.