His ratings have to be pretty replaceable at this point in Fox's business model, his radio show has always done horribly, especially considering he should be a "star" name. And they spent two years slotting first Megyn Kelly and now Tucker Carlson into Hannity's old slot and seen ratings go up, and Hannity's one of the top three radio ratings guys.
I figure they probably could shift Lou Dobbs over from Fox Business and barely notice, or move Tucker up, Hannity back to his old slot and just pick someone randomly from their thousand Fox News Contributors to hold down the last slot at least for a few months.