Having a different operator wouldn’t really resolve the problem of the infrastructure not being there to support more services.
Personally I wouldn’t want a situation where there’s a “bargain basement” operator introducing artificial demand patterns. It’s already enough of a nuisance with Peterborough/London passengers taking up space on GTR services.
Once upon a time there were proposals for WAGN, as it was then, to extend north of Peterborough. Had this happened then we may very well have ended up with a situation like is found on the WCML today, albeit I’m not sure where WAGN were thinking the rolling stock was going to come from, no doubt sweating the 365 fleet to the likely detriment of their core market further in. I’m really glad this never happened, the current situation with LNER operating an hourly stopping service is IMO adequate, although I do take your point that Retford is a bit of a poor relation in this, albeit a 9-car train every 2 hours isn’t too bad a service for a relatively small town.