I think TPTB really wanted to keep John Ross (and Omri Katz) in the show, otherwise they wouldn´t have repeated the scene of Sue Ellen leaving him at SF (Dream Season: "En Passant", Season 12: "No Greater Love").
Maybe it was a producers´ (=Katzman) choice to get "revenge" on Linda for quitting, as they did with runaway mummy Pam and Victoria. Or maybe the writers (=Katzman) thought that only one child at SF would turn Christopher into a weirdo (you know, imaginary friends and all that...).
"I´ll never leave you son, I promise..."
If "Dallas" had been "real", there is no way Sue Ellen had gone to England leaving her beloved son behind, after so many traumas and kidnappings she had to endure for him.
"I want to live with you Daddy. Mama is a drunk
and an unfit mother..."
And last but not least, the best part of that decision was the damaged version we got of John Ross in TNT "Dallas", probably one of the three best things of the sequel. Though they retconned it by making John Ross "obliged" to accompany Sue Ellen to London, instead of making it his own choice. Why did the Cider House have to ruin everything?
"This show sucks, doesn´t it Omri?"
"I´m not Omri...!"