The Great Central Railway London Extension was never really necessary. It was only built because Sir Edward Watkin had bought the Manchester, Sheffield & Lincoln Railway (MSLR), and simply HAD to have his own line to London, instead of paying other railways to carry MSLR traffic to & from London.
That is what happens when people consider the "free market" to be more important than sensible, logical planning.
Once built, it provided some useful links, and deserved a better fate. It could still have provided a useful service to some of the medium-sized towns between Chesterfield & Nottingham, but in later years, the services were painfully slow due to the effects of mining subsidence, and were allowed to wither away. As a route to/from London, it duplicated the MML. The only non-duplicated links were to Rugby,, Aylesbury, and to Banbury for Oxford and the south. Sadly, these were not enough to save the line, and it would now be almost unaffordable to restore it.