I think that to hang on to a car you need more than just 'I'll finish it one day.' I think you probably need to still be enthused by it and want it done for yourself. If your tastes/interests have changed and its no longer a car that you can see yourself driving there's not really going to be much point keeping it as its ultimately going to go to someone who does want it. Your own time is likely better spent on the things you do want for yourself.
(not aimed at anyone in particular)