Sorry, I'm not following.
It's not like the players can declare that there is a red dragon a point X. The DM does that. The DM tells the players, there is a Red Dragon here. Because of the level system, if the players are insufficient level, they cannot go there (or, well, they can but very, very likely won't). Which means that the players go off and do other stuff, only returning to this place once they have sufficient firepower. How is this not the DM more or less driving the campaign? I plonk down numerous locations. Some of them will be too dangerous for the characters. Some of them will be more or less plausible for the players to handle. So, I've now carved out a pretty clear path of "don't go here (yet)" and "here is a good place to go for interesting stuff to happen". The path of the campaign is going to follow those road signs pretty predictably.
My whole point for all of this is the argument that "sandbox allows so much freedom" is that this "so much player freedom" isn't quite as much as people want to claim. The truth is, the difference between a sandbox campaign and pretty much any other campaign isn't anywhere near as big as people like to believe. The amount of freedom for the players is going to depend so much on the DM that it's not really possible to say that sandbox campaigns allow maximum player choice.