In my extremely limited experience, to get press accreditation almost anywhere, you need to show some value to the accrediting agency. Mostly this just means being sponsored by your publisher, or presenting evidence of your published work. But for more important events, there is always an absolute number which is the size of the total press pool, with subdivisions in that.
So I would expect that, as well as having a press conference seat, the networks have a limit on the number of White House passes they get, and that number is subject to negotiation with the White House, because they are in competition with everybody else for the allocation of those spots. And in that negotiation, they have to justify why they need that many spots. And so “we have a spare journalist present in case one gets sick” isn’t going to cut it. Bottom line: they run on a skeleton crew.
I note that apparently, Acosta didn’t just loose his seat in the press conference: he lost his press accreditation. That means he’s not getting press releases in his pigeon hole. Those press releases won’t be going automatically to his camera man: his network needs to bring in and get accredited a new person. I assume that the WH would expedite that, in support of the argument that Acosta was banned for personal behavior, but they are in control of that.
In the event, I see that the WH has restored accreditation: I think that’s a recognition of what an organizational mess suddenly revoking accreditation causes. I’m sure that a big part of the objection of the press core wasn’t just about the freedom of the press, it was about things like job security and getting the product out every day, on time.