I think they could easily have made that flightiness/dottiness into a character trait that distinguished Jo from Sarah Jane without being so extreme as to prevent her from acting as a mentor. But Davies said in an interview that he thought just getting another old companion to replace Sarah would have been "so fake," and I can see his point-- everyone would always be very aware of what had happened and why. And it's not like the show was strangled in the cradle-- four and a half series is a respectable run.
Talking of old companions on SJA, there was also some consideration of doing a story where Sophie Aldred returned as Ace.