^^
You don't have contingency plans for everything they might do. What you have is contingency plans for everything they are likely to do
this session and a rough idea of what they might do next session. The players can only act on what you present to them, which limits things significantly.
My players understand plot isn't railroading if they choose to become involved. And the entire concept of every game should be an open world is bullshit. Most DMs aren't equipped for that. As a DM, I have a specific plot line laid out - maybe more than one, maybe with a bunch of sidequestd, maybe it's a massive dungeon crawl. I have room for leeway in this, but if I set out to run the red hand of doom an the players run off to the desert, I'm not ready or prepared for that. It isn't my fault when characters don't act like themselves.
Indeed, true sandbox gaming is probably beyond most people who aren't into improvised theater or professional writers. On both sides of the GM screen. Sandbox games tend to waver and taper off unless:
A) The world is sufficiently detailed to find triggers and pull them, as well as having enough predetermined events that will occur without the PCs intervention that it seems alive.
B) Self motivated,
cohesive players. Its all well and good for the characters to pursue their own motivations, but when they're haring off in five different directions even the best GM will be hard put to make it all work out.
I like using goalposts myself. Just note down key plot markers and check their status and likely consequences. This gives a lot of flexibility to tweak the details(oh so the PCs went to this town instead of this other location, sure, we can work with that), outright copy the details(the epic battle at the cliffside will now happen inside the church steeple instead) and keep track of the big picture(who's defeated, whose plans are in motion).
These work around the GM's human limitations, while he cannot create a perfect game, he can make the
appearance of a perfect game. Taken to an extreme in a sandbox, this just means that the places you visit ARE the same. You're destined to go through the encounters sketched out, you just run into them in difference times and places. Its very heavy handed, but easier to master.
The DM can also allow the party to split into two groups. When the 3 players that follow the plot get 3x the amount of time that the 1 player who doesn't follow the plot gets, he'll get bored soon enough and rejoin the party. Though I don't like any of my players getting bored, so I try to avoid this.
Not that great an idea. The player who's being left out gets WORSE, because its punitive, and splitting the party causes a big headache to begin with(time and locations desync). If he won't go find the plot, build the plot under his feet, and if he persists in avoiding the plot...well obviously this game isn't for him.