It's not that unthinkable that Kojima's grand ambitions ran into budget constraints and they had to cut down the story content. I can easily imagine dozens of more hours of voiced and animated cutscenes polishing out the content that's already in there, but voicing and animation are expensive.
What remains is imho still quite good and entertaining, I really enjoyed the open world parts, the base management, hunting for new soldiers or grooming favorites.
The multiplayer part made it all a bit grindy, without some story fluff in-between, but I think it was a smart game mechanic to add to a sneaking game where you have the option for non-lethal take-downs/straight up bypassing of enemies, especially in the context of the "lore".