I was involved in a meeting around two years ago which mooted the opportunity for six-car running between Doncaster and Liverpool all day but at the expense of Doncaster - Cleethorpes becoming a separate service but with a trade off with a half-hourly frequency between Doncaster and Scunthorpe at times which afforded a more clockface 30 minute interval service (memory seems to be that it was xx50 and xx15 ex-Doncaster and xx00 and xx30 ex-Scunthorpe but don't quote me on this). Plans were for one train per two hours for Thorne South, Crowle and Althorpe as now with an express in between, but no suggestion to extend beyond Scunthorpe.
Then it turned to TPE providing stops at Thorne South, Crowle and Althorpe every two hours which was quickly dropped by TPE as a viable solution and as such we end up with the status quo we have today.
Part of the problem is that the two-hourly stopper follows the express and in the opposite direction although xx48 from Scunthorpe arrives just 10 minutes before the express anyway which partly explains the severely poor passenger numbers. Only when TPE is late do you see a reasonable load on the stopper.
There's room for improvement here and back in the old days there was a decent xx08 to Scunthorpe with the xx42 (as was) to Cleethorpes and xx10 and xx19 to go to Doncaster [not ideal], but that was back when the service ran from Sheffield which was removed to make use of platform 0 at Doncaster and alleviate crossing Marshgate Junction.
The current situation really doesn't work and needs SYMCA, NLC and NELC to propose something better, but better comes at a significant investment and we can guess the rest.