Dunno.I may have seen that worry in some NPs I know. I was reading a post from one INTP on the forum, saying that he constantly regretted alternative routes in his life that he didn't take. Do you think that relates to it? That it's a fear of creating that certainty, and making those decisions that you can't turn back from, because it removes the safety of the huge number of options that were previously available? Not sure if that was just the particular INTP, or whether it's a P problem, or a Ti problem.
It might be a bit restrictive to work this stuff in terms of functions. I think it is true that there are times when what we call this function or that is more active than others, and the focus of activity does change. But it might be important to think in terms of wholes. Like, I think if the inner world is being consciously accessed, then both inner judgment and inner perception are relatively more active than the extroverted pair, but some aspects of extroverted content must still be present, just less dynamically so.
Perhaps this is some idea of seat of consciousness. (Or half the seat, anyway: the inner half. Maybe)