I don't think I've ever completely lost interest in a game I've actually paid for without sinking a decent amount of time in to it, though I will say I picked up Assassin's Creed 3 on sale earlier this year and it was a real struggle to push through it compared to the other games (I'd played all the other up to and including Black Flag). Just didn't really hold my interest for more than a few hours every few days.
Actually I never finished most of my final fantasy playthrough. The one I've finished is FF4 remake on DS and FF9 on PS1. I still have the save files on my memory card though. I just kind of get bored and forgot about them. I've played FF1-FF9 and FF13 on PC. (Don't have PS2 or above console, though i still have my SNES and PS1)