Rahm's audience isn't city residents (most of whom won't use this train) but companies that might choose their HQ's partially based on such considerations.
That's useful to foreground and I agree that's his target. There is a reasonable argument to be made that a constellation of policies targeted at courting big businesses will bring more of the likes of McDonald's, Caterpillar, Boeing, etc to downtown Chicago, and this will help lift all boats. Meanwhile, it's unclear how much finite political capital and staffer attention this project burns that could have been, eg, directed towards tracking down funding for the Red Line expansion. Maybe they can walk and chew gum. More likely they can only chew like three sticks of gum and I wish this weren't one of those sticks. (Not that you were making a claim either way. I'm just riffing on your post.)
The Blue Line isn't a train to the airport, it's a commuter line that just happens to have an airport at one end of it.
Airport-bound passengers have luggage that takes up space, blocks aisles and seats, etc. Moving them to a dedicated airport train frees up a lot of capacity that city residents will happily accept and appreciate.