I think what MGS5 offers the player is immersion as a tactical action game. They could have applied a Delta Force or Ranger backdrop and kept most of the gameplay intact and it would have felt good. It doesn't really matter how accurate the game is...it's about that feeling you get when the helicopter drops you off and your life depends on staying hidden. Previous MGS games were more about the absurdity of the story line and bouncing from cut scene to cut scene.
I think if you removed the story, dropped in a quasi real world scenario, and cranked up the sensitivity of the troops...there could seriously be some magic there. Then again, I'm a masochist that plays Operation Flashpoint games.
It's somewhat like Assassin's Creed Black Flag which is a fun pirate naval game wrapped in a cheesy plot. MGS5 is a fun, immersive special forces game wrapped in a cheesy MGS plot. I think this explains why people put so many hours into it. Everyone wants to be Chuck Norris.