In theory an emergent GM could predict what the players would do and prep accordingly while letting them make decisions.

In practice, I have doubts. Once you start designing something, it's kind of second nature to prep the path to it as well. It's also incredibly easy to start making other things subtly harder or putting up resistance, so when players see that, and see the prepped path, they start just looking for the path.

I'm not saying it's impossible - but I don't think I've ever seen it executed on well. It's also the kind of thing that from the DM's side can be hard to detect, since how do you know if you predicted players, or steered them onto that path? A lot of times players expect a path, and look for the path (see examples in this thread and others). From the GM's PoV, that kind of implicit participationism looks a lot like correctly guessing what the players will do.