Good question.

I don't think central planning vs. distributed decision-making is relevant though, because it seems to me that either way humans make decisions similarly much: the question is just whether it is a large or a small number making decisions, and who decides what.

I usually think of the situation as there being a collection of (fairly) goal-directed humans, each with different amounts of influence, and a whole lot of noise that interferes with their efforts to do anything. These days humans can lose control in the sense that the noise might overwhelm their decision-making (e.g. if a lot of what happens is unintended consequences due to nobody knowing what's going on), but in the future humans might lose control in the sense that their influence as a fraction of the goal-directed efforts becomes very small. Similarly, you might lose control of your life because you are disorganized, or because you sell your time to an employer. So while I concede that we lack control already in the first sense, it seems we might also lose it in the second sense, which I think is what Bostrom is pointing to (though now I come to spell it out, I'm not sure how similar his picture is to mine).