I would concur with AC on this to be honest. They sold you a pass to Europe. Even though your intention was to go to Istanbul the pass still will get you to Europe.

On the other hand, I do find it abhorrent that almost all airlines can sell you a ticket for $100 but you need to pay $25 to change and $50 to cancel. They haven't sold you anything prosaic or tangible, there is nothing physical to return, simply future travel (which these days isn't even guaranteed).