I know Phoenix has had adversity behind their team for a lot of their time in Phoenix but it's just a bad setup overall.
Attendance percentage by year (asterisk = playoff year)
77.5%, 80.4%, 81.3%, 72.3%*, 71.2%*, 68.5%*, 85%, 84.7%, 85.6%
Sure, McDavid is going to add bums in the seats in Phoenix but I think the notion that Phoenix is going to be anything more than a half-empty arena along the same lines of Florida. Pittsburgh wasn't saved because of Crosby. They were saved because Crosby + new Arena + Lemieux allowed them to tap back in to a solid fanbase.
And besides, the Leafs didn't increase ticket prices this year. If they somehow win the draft lottery and draft McDavid you know full well that there will be some kind of absurd 15% increase across the board that would likely outdo Phoenix adding 2000 fans a night through their buy 1 get 7 with hotdogs, parking and 1 month of your condo fees getting paid off.
Mind you McDavid would immediately become an overrated bum if he was drafted by the Leafs so it's probably in his best interest that he goes elsewhere.