Right - that's why I said it's selfish. and sorta unlikely, too (my corollary to myself)
yes its technically selfish, but its the nature of the beast. better league offers you to come in, you go if you feel its right for you. its on nycfc to not have plan B's or ways to convince them to stay. i do recall there were rumors ( twitter though so take as you will) that St Etienne was looking for him the year before and he said no. so nycfc should of known or even talk with him and have a plan if there is a chance a team comes calling for him in the following summer.
just like in any job, better job comes around you go ahead and take it, give two weeks notice and you on your way.