The NYMR does quite well out of people making a day of it in Whitby. The length of rhe train ride is less problematic when you're having a day in the town and people are happy to pay for a steam ride as a novelty.
Different to many heritage railways where people will generally be spending the day on the line and want time to visit the sheds and museums etc.
I think when families make the trip some of the group will be looking forward to the steam train ride and other to the day in Whitby, maybe even splitting up and doing different activities depending on interests, so a family group can have a day out that ticks a number of boxes. Had the line from Pickering to Rillington been saved how many people would use the Malton/Pickering section, not many I suspect, Malton is not as tourist/holiday oriented as Pickering. You mght capture some day trippers from York, but to be honest if you are doing the journey by public transport from further afield you will use the Coastliner bus which is one of the better bus operations with a simple interchange at Malton. The let down will be the dire TPE 'will it run' lottery into Malton, partcularly on a weekend.
The NYMR are looking carefully at finances anyway, and looking at ways to operate profitably, as costs seem to be rising quicker than income. I am not sure that a further 6 or 7 miles of route would help.