Mm, but if the Cyberman's claim that "we are inevitable" is correct (i.e. that this is going to happen regardless) ie either of Group A or Group B will wind up in the horrific situation, then it's reasonable to prioritise the currently-existent Group A.

And in particular, time-paradox-wise, then if the Doctor's companions don't exist, then she doesn't wind up in the house and able to save Shelley anyway, right? So there's no one to stop the Cyberman from getting what it wants.