Right, but you're missing the part where in the real world, nobody on the plane is going to carpool with other passengers they've never met, they're going to drive themselves in their own car, because people are not automatons that instinctively select the most climate-friendly option.

We shouldn't be comparing the most optimistic outcome in the world as you'd prefer it to be that could happen as a result of the flight being cancelled, we should be comparing the most probable outcome in the world as it actually exists.