Talking of the MML (and sorry for veering slightly), but why was the very modern (at the time) Great Central Line closed?
The Midland Sheffield - Nottingham - Leicester route serves a number of other towns as well, whereas the GC went through sparsely populated countryside.
The decision in this case was made to retain the Midland on the basis that, although slower, it made sense to concentrate on the route that served more places.
Meanwhile, in another part of the country, a decision was to be made regarding retention of either the GW or SR routes to Exeter, Plymouth and the west.
In this case, the SR route (like the Midland) also served a number of smaller towns on its route, whereas the GW (like the GC) went through sparsely populated areas (eg the Berks & Hants cut-off).
By contrast, he decision in this case was that the SR route was to be run down, because (the argument at the time went) serving all those towns slowed services down; the route to be retained was the faster one.
It's also worth pointing out that, at the time, the GC had not long passed from Eastern Region to London Midland Region control, and the Southern lines west of Salisbury had not long passed from Southern to Western Region control and in both cases the line retained was the one historically associated with the Region concerned.
I will refrain from offering any opinions myself, but leave the facts to speak for themselves.