As far as the Iberia comments. I have to say, I made the mistake of flying Iberia to Madrid once (From Logan)and it was horrible. Uncomfortable, Unfriendly and it certainly wasn't 100% full. A fluke, right? no... my return trip was equally as bad. When my sister's friend from Madrid came out to spend some time with her, she flew Iberia and complained about the poor service and lack of comfort as well (and she is all of 4 foot 9) and commented on the lack of passengers.
I would say that it's perfectly reasonable to think that that Airline may leave the city. My last two trips out to Madrid I booked on Lufthansa via Frankfurt and even that was much better. In my brief international travel experiences (Madras, India 2 times on British Airways via Heathrow, Amsterdam once, on Singapore Airlines out of Newark, Germany once, Frankfurt with Lufthansa and Madrid three times once on Iberia and twice with Lufthansa via Frankfurt) Nothing has been comparably as bad as Iberia, and from what i hear, I'm not alone.
I bet those people are choosing different routes (Hell, my mother goes down to JFK to take Delta to Madrid instead of Iberia), because any other European trip to/from Logan has been full in my experience.