Let's keep moving the goal posts then.
Per TrueAchievements.com, only 20% of total players have earned the "The End" achievement on Minecraft: XBOX One Edition. This is the achievement earned after defeating the Ender Dragon, ostensibly the game's final boss. That is what would count as completing that game, a game which isn't story driven whatsoever. Only 9% of the open-ended no-narrative sandbox game Cities: Skylines players on XBOX One have unlocked the "SIMulated City" achievement, earned when the player unlocks nine map tiles and reaches that open-ended game's "late game" state.
A better frame of reference: single-player sports game modes don't do well speaking to "completion" in the general case, either. 12% of players have completed the "Man Of The People" achievement in NBA 2K17, earned when one reaches 5,000,000 fans. 2% of players earned the "How Much Ya Got?" achievement, earned upon signing a max deal in MyCareer. Only 34% of players have earned the "Show me the money!" trophy in MLB The Show 17, unlocked when a player signs his first MLB contract in Road To The Show mode.
Most tellingly, speaking to other sports games with focused story modes, let's look at FIFA: only 32% of players achieved "The best is yet to come" in FIFA 17, unlocked when completing the story of The Journey. 16% of FIFA 18 players have earned "What a comeback", unlocked when completing The Journey's story in that game. A less relevant data point, but another sports game with a story: only 16% of players unlocked "The End" in Blitz: The League, earned when completing that game's campaign.
In comparison to all of the above, 44% of players have earned the "Complete Act 3" achievement in Madden 18, unlocked when the Longshot story is completed. By that measure, Madden is doing incredibly well for a single-player game. People who play Longshot engage with it, as EA has publicly boasted, and more importantly finish it, as proven by the number of people earning this achievement. For the record, I fully anticipate that this number goes down next year in M19 when Longshot is less of a novelty, but for now it's doing very well.
Sure, there's validity to the point that a story mode isn't exactly expected in a sports game and that would decrease the completion rate. That's obvious based on FIFA / NBA 2K / MLB's numbers. However, there are
countless examples of people not finishing story modes in single-player games, whether narrative driven or not, enough that it's an industry-wide trend and plenty of people in the industry talk about how to increase that number because it's stupid low. There's also the fact that Longshot in M18 is doing alright in the first place.