The McGill's Airport Express will always have demand. As I've said before, it covers a stand just outside a completely separate arrivals hall. Whenever a flight arrives from there, people will go to the exit and have an orange bus sat with 'city' on the screen right in front of them. In the same way, for the other two arrivals halls, you exit and the tram and Airlink are sat right in front of you. At the Airport, unless you've planned in advance, or have past experience, the choice is almost non-existent. Most just want to get to the city. The mentioned arrivals hall is quieter, so unsurprisingly a lot of Bright Buses do operate quiet or empty, but not all of them, though perhaps I'm unlucky in my observations as it seems to be most of them. Evidently, the ones that fill well make enough to justify operation of the service. Locals may have loyalty to or familiarity with Airlink/the local transport provider, as is the case with me.