Airport workers in general are unlikely to take a train from the downtown to work, because they don't live in the downtown. There are broadly two groups of jobs at the airport; highly aviation specific ones (e.g. aircraft mechanic, Customs officer, pilot) and really generic service jobs (e.g. Tim Horton's or cleaner). If you're working in an aviation-specific job, you will want to live near the airport because that is literally your only possible workplace in the city. (If you're a lawyer, in contrast, there are any number of law firms, not all of them downtown.) If you work at Tim Horton's, you are likely to live near the airport because there are jobs like that everywhere in the city and if you live in Cranston, why would you drive past 100 Tim Hortons to get to the airport?
So the bulk of airport workers live in the northeast or north centre, and there is not really any fare low enough to justify taking the C-train 15 km downtown just to transfer to an airport train to travel 15 km back to the job that is only 5 km from your house. A people mover type system that connects to transit nodes in the northeast and north centre on the other hand...