First, I said they (whomever replaces Beck) will get about the same ratings. Look at Beck's most recent ratings. He barely outperforms Bret Baeir and Cavuto. His replacement may suffer some, but I rather doubt that Beck's ratings will be halved, or worse - which is what it would take for FNC to not win the time-slot.
As for Gibson, he wasn't fired. He did not want to continue his daily television show anymore and do his radio show too. So, he picked the radio show. FNC was happy with his ratings, as he always comfortably won that time slot.
I have no idea how Ingraham did in the ratings. She was only in that slot for a trial period, and she just didn't do very well in that kind of role.
Gibson was decent, although he didn’t uncover nearly as much as Beck. The fact is, anybody FNC puts in that slot will crush Matthews and Blitzer