That line is really slow, it can be faster to get to the airport by getting the tram to East Didsbury then changing to a train.
As a means to get to the airport from the city centre? Yeah, heavy rail is probably better.
But that's not the only reason for the line. It's used to get to and from other points on the route too.
The Airport Line might be better labelled as the Wythenshawe Line.