Have Grand Central not tried, or not been allowed to, issue tickets with Northern connections? It's a genuine question.
Grand Central will continue as long as they are competitive on time and price with other operators. The Bradford trains out of Kings Cross on an evening are busy, usually full and standing to Doncaster on Fridays at least, as they are competitive on time and price. Loadings to Halifax are usually very healthy.
I use Grand Central Bradford/Halifax-London as they are a direct train and they are competitive on price. I'm sure most people are the same. I may be seeing Yorkshire at a snail's pace but I can spread out and get comfortable, something which I can't do if I have to change at Leeds.
But I do suspect that if the Doncaster stop was removed the service would not be viable. Just like the Sunderlands, if York was not a stop. In other words both these services need the ORCATS raid to survive.