O'Reilly is an alpha male. He appealed to the older male demographic that is Fox's bread and butter. His competitors on MSNBC and CNN were a lesbian and a homo, so he did not face any real competition for his demo.
Fox's only other fulltime alpha is Eric Bolling. He is something of a novice at politics. To retain O'Reilly's audience, they need another alpha or a smoking kickass girl.
It appears the Murdoch boys are set to make what I would call the NASCAR mistake. That happens when consultants convince you the way to grow is to broaden your appeal beyond your traditional audience. To do that, they advise, you have to smooth off the rough edges and become less controversial. Unfortunately, it is tough to pull that off without losing what attracted your audience in the first place. You end up with a boring, generic product that annoys your legacy audience and fails to register with the demographic you are targeting.
Having stupidly shitcanned their bellcow, the Murdochs now face a real crisis. Their first attempt at solving it, sticking the Five program in the 9:00 slot, seems laughable. They either need to find a credible O'Reilly replacement or go way outside the box in a move that will generate a lot of buzz. I just do not see another O'Reilly, hence I say go with a young girl like Tomi. Sink or swim, at least it won't be boring.
They could swing for the fences and go after Limbaugh. He brings his own huge audience, but would he do it? Maybe, but the Murdoch's seem to want to get less controversial and edgy, not more.