It is completely fair to have story completion as a requirement for a singleplayer game.
In fact, that should be a given and not something to be considered as "imposed", as you state it. That already says a lot about your attitude towards the game.
9 hours for a 30+ hour game is laughable to make a final judgment.
If anyone feels different, they definitely shouldn't be a professional games journalist.
Also, nobody cares about people giving low ratings unless it's unfair.
So, if 80% of the professional reviews give 80+ scores, a 3.8 or whatever the lowest score is, is too drastic of a drop. IGN's 6.8 isn't that far fetched, tbh.
The only reason people are "up in arms over any negative comment", is because said negative comments completely dismiss 82% of the reviews and accuse those reviewers of being paid, while only validating the one 6.8 and one 3.8 reviews to push their hate campaign against Kojima or PS4 exclusives.
The majority of people in here are level-headed and reserve judgment until they play the game themselves, instead of certain users who carry their pitchforks into every single DS thread for the past week.
Guess objectivity is too much to ask for on forums.