Having looked at this in some detail,
Goodness, you have!
I've a lot of respect for this - it's a plan based on the practicalities of working the line and the resources that are likely to be available. Even as a strident supporter I think it's important to be realistic about the money / service level that's justified. Brigg and Kirton aren't teeming metropolises and they're never going to fill an hourly nine-car Azuma. A basic operation is all that's needed. Heretical as it sounds, I might even say that a stable weekend-only service could be better for the needs of Brigg & Kirton than a weekday one if we had to choose.
If the following could be satisfied:
- Reliability. Only the most determined passengers are going to plan journeys when they know there's a ~30% chance their train will be canceled with a wait on a barren platform of 3+ hours for the next one. Particularly when there are no announcements at stations, no information screens and the operator (in contravention of the Passenger Rights & Obligations legislation) tells stranded passengers they have to source replacement taxis themselves.
- Sale of Advance tickets not broken. See below. (to be clear I'm talking about long-distance journeys, I don't expect a cheap Advance to Barnetby and back)
- Promotion, maybe with the odd special offer to coax locals onto them. This could even be dressed up as something nice, a relaunch of the service.
...then I think a basic operation could be a worthwhile endeavour. It's never going to make a fortune but the operator might well break even. If they do that for a year or two and usage doesn't pick up - well, then they gave it a fair chance and closure - via the correct legal process, not the back door - will be easier to justify.
One minor suggestion I might make is that given the nature of the line's usage, a better service would be justified in summer than in winter. I don't know what that would look like - perhaps an extra train late on Saturday evening for people who've gone out to Cleethorpes? Cleggy has some good pubs these days so if properly promoted I could see it being a hit. Other rural routes seem to have found success doing similar.
A mediocre timetable is doable
Here in Brigg we're used to mediocre timetables

Send us a train, any train, even a clapped-out old Pacer. So long as it turns up at vaguely the advertised time and there's a way to make a return journey we'll be happy. Just as you say, a mediocre service is better than none. Connections from the soon-to-be-ex Saturday service were seldom great[1] so expectations from passengers are not high.
Sadly there's an elephant in the room - the operator itself. Northern really doesn't like the Brigg line. The management seem to see it as an inconvenience, with usage by the public not at all to be encouraged. When Northern stonewalled my complaint a couple of months ago I put in a few FOI requests to see if I could get the true story and the results have been fascinating.
Northern has thus far refused to release anything (citing commercial sensitivity, which I hardly think would cover every single document and email it possesses) but the DfT released
this document - seemingly a presentation made to it in June 2022 by RNP citing Northern's desires for the route. The proposal within it (totally the opposite to what Northern told me this March about it having no plans to change the route!) was "Remove Includes closure of 3 stations – Brigg, Kirton Lindsay [sic] and Gainsborough Central". Going by the document the transport minister was briefed (or intended to be briefed?) ahout "Complete closure of 4 stations at Brigg, Gainsborough Central..."
As far as I can tell, the only reason Northern didn't proceed with total closure at the time was that the Rail North Partnership (who are a combination of the DfT & Transport for North) [2] talked them out of it. These are not the actions of a TOC that wishes to build up a viable service. No wonder Northern wouldn't answer my questions when I complained.
[1] An unhelpful side-effect of this is most journey planners haven't been showing Advance fares to Brigg/Kirton even though they do exist. I think they get confused by the infrequent service on the line and instead try to route passengers on a quicker journey for which only a full Single is available, costing 3-4x more. That isn't anyone's fault in particular, more a limitation of the booking engines TOCs use, but it does contravene the NRCoT requirement to offer the best value ticket and ought to be fixed to encourage use of the line. No-one's going to go to Brigg when it costs £95 and you could go to Barnetby for £30!
[2] See TfN's
FOI response here. It's a real credit to them - they have a positive attitude and want to give the line a chance. I don't think they're the problem - more a disinterested operator that has to be pushed into running any kind of service whatsoever.