There's definitely a lot of options on MXP-JFK just due to the huge O/D traffic, and better options than ITA at that, which is probably why they found the route unprofitable and had to back out. But for a lot of other destinations this isn't really the case, and they could make use of connecting flights too. Just as an example, AC flies to MXP only from YUL (from YYZ it's a direct flight technically but stops at YUL anyway so I'm not counting it). So ITA (and hence AC bc of the JV) might benefit from a MXP-YYZ taking some of the load off the YUL flight.