It goes both ways; there's a feedback. People start watching a channel because they're predisposed to agree with it, but when the channel constantly reinforces their biases, it foes from a slight predisposition to a strong opinion to an absolute certainty.

Fox News doesn't just give a conservative opinion on events, but constantly asserts that every other information source is wrong. Not just wrong, in fact, but a deliberate attempt to con them.

You're correct that eliminating Fox News would not, in itself, end that process. They've had decades to reinforce their views. It may well be inescapable at this point.

But OANN isn't as slick as Fox News. It doesn't attract people with a predisposition; it's more likely to turn them off. If Fox News were to disappear, and OANN expand to fill its space, it might eventually reduce the number of people drawn into that self-reinforcing mechanism.