What does it mean to complete a game?
It means different things to different people that may even vary for different kinds of games.
Are you a completionist?
Do you hunt trophies?
Do you re-play games?
When a game has branching paths & multiple endings...do you *have* to see them all?
How much free time do you have for gaming?
All of these different kinds of gamers would answer differently depending on varying situations = there is no *RIGHT* or *WRONG* answer here.
It's like asking: "What constitutes a RPG and what are the differences/criteria betweem WRPG's, JRPG's, SRPG's, etc ==> You'll never get a clear & concise answer -and- different people w/ differing opinions will argue & debate it and the discussion will never end