I guess if you did reopen it you would have to extend the Manchester to Buxton service to Millers Dale so you could actually get to Buxton on it.
Manchester, Stockport, Chinley, Millers Dale, Bakewell, Rowsley, Darley Dale, Matlock and stations to Derby would be popular in summer but not so much in winter.
The hard fact is that we have to find a way of getting a substantial amount of freight/parcels back on to the railways to make such routes viable, as finding someone to pay for a reserved trackway for a two car train every hour and nothing else is always going to be a struggle.
In the meanwhile a grant to help extend the heritage line to Bakewell would probably be the best course of action.