Not right now obviously - the PAC is behind the Big 12 at the moment. But the PAC has some large markets and if they have a run of more success on the field and the Big 12 struggles, then the PAC could move into poaching position for some teams.
Is that likely to happen? No. But could it? Yes.
Truth is that it's unlikely that both those conferences survive long term. The Big 12 is better positioned for it to be them...but things change and it's close enough that the PAC could pass them in certain circumstances. And that would be bad for BYU.
I don't think either is strong enough to kill the other right now. But I do think it's likely they are on an inevitable collision course at some point in the future