As for Alfreton, surely stopping in the peaks might be better, InterCity services can be caught from Nottingham or Chesterfield, and the Northern stopper allows an interchange at either end, and so it's mostly just commuters left using the service beyond Sheffield or Nottingham I imagine. Of course idealy the line would have 3tph, of which 2 stop...
Alfreton generally sees more patronage out of the three stations along the Erewash Valley; when you compare Alfreton to Ilkeston and Langley Mill which only sees an hourly service off-peak with the Norwich to Liverpool's stopping during the peak as well as the HSTs coming to and from Leeds, Alfreton is better connected across Derbyshire and parts of Nottinghamshire with comprehensive road and bus networks. Reducing the off-peak service at Alfreton maybe difficult to justify compared to Langley Mill and Ilkeston.
EMT withdrew the regular off-peak services from Langley Mill, only maintaining a peak service when Northern introduced the stopping services from Leeds to Nottingham, whilst Alfreton maintained a twice hourly service owing to its higher usage and better access from the Amber Valley and parts of Nottinghamshire.
When you combine bus and rail services from Alfreton to Chesterfield and Nottingham, it is night-and-day! Alfreton to Chesterfield, bus and rail services is comparable in journey times, but rail services is superior from Alfreton to Nottingham. An half-hourly bus service to Nottingham takes 1hr 20mins, compared to rail which takes under 30 minutes, with the Northern service taking roughly 5-6 minutes longer than the fasts by EMT. Reducing the amount of services makes it less attractive for passengers to use.
Of course, this could all change when Norwich to Liverpool's is split by 2021 under the East Midlands ITT.
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(I'm trying to make a joke)