I personally don't think it's "unfinished" although it MIGHT have that feeling and I think that theory is kinda reaching and absurd but... I feel like it was intentional. I honestly don't find it likely that he "rushed" it. Mission 51 was probably intentionally cut, in my eyes, despite being only 30% complete. It adds nothing more except closing a plot line. It adds nothing more to any of the characters or story at all besides ending the plot line and we all know Kojima doesn't make stories to complete the timeline. He does it to tell A STORY. Although, I'd love to have 51 here, I just believe it was Kojima's decision to cut it whether it was because it's "pointless" or to make the player feel "the phantom pain". I kinda feel like the replays of missions are there to strike an entirely different point....just another day in war. Outer Heaven isn't fun to do work for...it's tedious. Doing it over and over again, in endless war, isn't easy nor is it fun. I think Kojima may be stressing this point. "Just another day in Outer Heaven" doing contracts for MB just to expand it...doing war over and over again and seeing the "what's happening at Mother Base" cutscenes and if you think of Chapter 2 as an epilogue then it's much more digestible. You also don't have to do them so there's that. Venom refuses to stop because he can't change his fateful path to death that was set for him. Him cheating death with Big Boss will haunt both of them. The Phantom Pain is about characters who refuse to change...Venom, Big Boss, Miller, Eli....and it all leads them to their inevitable deaths.

I know this isn't organized very well but this is some of what I think. I know there isn't much striking evidence to it but here's me.