I don't see why there should be any rush? How many wingers do the Leafs need, exactly? There is no risk in developing him more patiently. There is risk in rushing him to the NHL, and the potential reward is mostly appeasing rabid fans.
People said the same thing about Kadri, going as far as to invent conspiracies by coaches against him. Ignoring that each time he got called up he was being physically dominated and spent half his shifts getting back up off the ice, from which he probably didn't learn anything valuable. Some fans just need to have the newest, shiniest thing right now. There are many more examples of players flourishing after patient development than there are of players excelling in the NHL at 18 or 19 years old.