A game is compromised of everything, not just the gameplay.
And if you are playing a STORY based game, then the story is the most important part. It's literally to move from one chapter to the next. To learn more about the story.
If you just want good gameplay then play something that is entirely about that.
In all fairness, the gameplay in MGS4 is pretty damn rad, it's just there is not enough of it. It's definitely good fun, too bad we never got the equivalent of like PS1 VR missions or MGS2 Substance.
Imagine infiltrating Shadow Moses with genome soldiers and recreating elements from MGS1 or night time missions in chapter 2, advanced stalking missions in chapter 1, survival waves in chapter 5, film noir detective missions in chapter 3