I don't completely agree with OP.
I didn't care about the plot, or the NPCs, or the boss.
But MGSV succeded at hitting me hard on several occasions : the conclusion of the GZ prologue, the epidemic outbreak, and especially that secret conclusion with Snake facing his reflection in the mirror for how it loop back to the very first MG game.
These are basically the endings.
Ground Zeroes solidifies that Big Boss is at his most villainous through horrific trauma. His former friends and pretend child soldiers are having bombs implanted in their bodies while Big Boss hides nukes from world authorities. The disastrous night is like him hitting rock bottom. The trauma and loss that Kaz mentions time and time again will always stay with them.
most of Phantom Pain is about that trauma. PTSD and the effects of war. Venmo’s introduction with the doctor and slowly realizing how fucked he is and discovering his arm is missing etc was all incredible and the kind of storytelling you can only do in games.
I liked how the effects of PTSD continued, how Venom kept seeing things he didn’t know were real or hallucinations. Some people demand explanations for all of this, they are missing the point. Kojima is showing what war and guilt does to a man’s body and mind. The Paz stuff in TPP in particular has him grapple with actions that fateful night.
On that theme I like the use of furry Volgin as literal Revenge, a supernatural force that works for whoever’s will for revenge is strongest. He is attended by Psycho Mantis, indicating that the kid has very intense feelings of revenge.
But revenge is self destructive. Venom gets his revenge on Skull Face but in doing so he takes on the metal gear as well as the parasites, inheriting Skull Face’s stock of world threatening WMDs. At this point where is there to go? Any new characters introduced would have to be killed off since they aren’t part of the other games. Venom has built the Big Boss army but he has to prove himself capable of taking down Solid Snake, of killing his own fellow Diamond Dog, something he forbade his troops. This is thing that separates Snake from Big Boss in MGS3. Named Snake earns that title after killing the Boss, his fellow soldier. Venom like all Bosses has to do his turn as a villain, to serve the greater story.
This is why the Shining Lights mission is so impactful and it ends with him on his knees, utterly defeated. The next mission unlocked is The Truth, and I take it that the whole experience of killing his own men was so traumatic that it caused a breakdown resulting in the realization of The Truth. Quiet leaves Mother Base around this time and her departure means he has lost the first person he remembers seeing after waking up. Well aside from “Ishmael” of course.
People say there is unfinished story but I don’t see it. Big Boss wakes in 1984 and battles his enemies building his army over the next decade until the conflict at Outer Heaven. We see that hinted at in this game, and I love the way it is done. but if you were hoping this game would also include a remake of the first Metal Gear then your expectations were unrealistic.