I'd defect. The first person to defect wins the entire pool. It's not like any agreement among the passengers would be enforceable.

That isn't how game theory works. Colluding with 100 people becomes harder the reward for defection grows higher.

And so does the incentive to try shenanigans that break the simple model of the game.

There's nothing to suggest the model is overly simplistic. Good luck convincing 100+ people to collude on those shenanigans at their own personal loss.

If your model says that 100 people never stick to an agreement, then yes, it is too simplistic.