I wonder if Hulleys saw the popularity of TMs 218 and tried to emulate that. The X70/170 though doesn't serve Chatsworth. Doesn't serve Chesterfield Railway Station for easy connections, and Chesterfield doesn't have the catchment area of Sheffield.
I agree with all your points here.
Additionally, as discussed in the X1 thread with a few notable exceptions there does seem to be a policy that bus route numbers aren't displayed on bus stops up Chatsworth Road in Chesterfield (this is the case for all routes that run this way), therefore, the casual bus user who might use the X70 to pop into town won't know it exists at the moment until they physically see it pass the bus stop.