No doubt that the intro fare is extremely good and LAN is a quality carrier to travel with. Time versus money (and visa issues) will obviously play a role for several customers. Total elapsed flying time YYZ/EZE :

LAN 19 hrs 10 mins
AC 13 hrs 40 mins

LAN's true success on the route will hinge on competitive service in terms of elapsed flying time and think using JFK as a "tag-on" city will prove costly in the current economic environment. Given the duty day, would assume that yet another added expense will accrue with the crew laying over in New York, operating the JFK/YYZ/JFK shuttle, and laying over again in New York before heading home.