Good question. Why not use the crosstown for airport service? I suspect it has a lot to do with some hidden clause giving Blue 22 exclusive access to the airport. A crosstown GO line from Agincourt to the airport would have significant demand. Instead, the MO2020 gives us crosstown service from Agincourt to Weston.
I don't think familiarity is an issue. Certainly, Metro residents aren't apt to use GO on a daily basis. But I am sure that if the service was there you would find riders quickly. As it stands, GO today is not the most convenient service inside the 416. And then there's challenges like fare integration, service integration with TTC surface routes, etc. Fix those and more people will be taking GO. This is what leaves me disappointed about the crosstown being planned as a commuter instead of a core service. That won't resolve any of those issues.