Oh yeah I mostly was considering the idea of it being an evaluation of the narratives ability to make someone want to see it through, due to the variety of things that make up a game this is also competing with a bunch of other factors so not exactly a perfect metric. but I can at least see how completing a story can be indicative of its narrative strength (given considerations for general game difficulty and gameplay style) whereas completion has almost zero connection with the strength of the gameplay loop, which makes it a particularly odd choice imo for describing how well something works as a game.