I feel your pain, it's a real shame to see it go. Out of bitterness I could laugh and say "if we can't have it no-one can" but I really wanted to see Belfast keep the connection, just as I want to see Bristol get theirs back
Maybe we need an operator like Jetblue, and A321neoLR's to make thinner transatlantic routes work? I know for Newcastle at least, United were occasionally too expensive (read: roughly the same price as BA/AF/KL/EI, so people just went with them instead)