It would have been realistic to have maybe 2-3 background survivors survive with the main group, but the whole bus thing was meant to kill all the remaining prison extras so we could focus on just our main/core group of survivors as they start the next chapter (post-prison).
I kinda agree with User:174.2188.8.131.52.,that's why when i imagine it,there was more prison survivors that escaped with Emily (a fictional character i made up) and her brothers,Billy Greene and a couple who were welcomed into Rick's group before the woodbury situation
I Love background people for some reason but it seems like they always die. And what happened to Jeanette.
The actress was not on set for filming, probably to save money, after the directors decided there were too many extras on board (Denny Ainsworth and Hannah Moore were also removed) - but Jeanette would still be considered dead as those who were not in the scene were informed their character had died. We do see a few bloody corpses on the bus so it can be assumed some people were eaten and not reanimated.
A few minor named characters in the group would've been kinda neat. Just random survivors. No real importance to the plot. Lots of the redshirts in S4 felt more like a burden for the main cast to look after than actually being a part of the group.