But what else could it really do? There's only so many stories that Metal Gear can tell. There's only so many "OMG THIS NEW METAL GEAR WILL RULE THE WORLD!!" There's only so many things that the series can tie up at this point, and the last thing it need is MORE loose ends.
The Skullface story was simple, and something like MGS3 is leaps and bounds ahead of it, but that's kind of what this game needed. It didn't need to give us a big complex story that could potentially fuck up the timeline even more. Comparing MGS4's climax to MGS5'S just doesn't seem fair. 4 was attempting to resolve the entire Metal Gear story, where as 5's was just to give more context into what Big Boss was doing at that time and Mother Base/Outer Haven.
Honestly, the Ground Zeroes incident alone would be enough to turn Big Boss into the "demon" we know in MG1&2, and enough to turn him against Zero and Cipher even further to the point to where his state in MG1 makes sense. That is why I like the twist myself. Like another poster stated, put a great emphasis on "legends", but more than that, it shows that Big Boss has already made up his mind. He doesn't need to go through more shit to become the villain he was in MG1&2. He already killed his mentor and possibly his lover, was put through hell in Costa Rica for attempting to help and had to go through watching his mentor die again as an AI, was attacked by Cipher from the inside, and then once AGAIN later, the second time having his entire base and so many soldiers lives lost. All of which was orchestrated by powers beyond his control, no matter how strong he became.
Because of that, the twist makes a lot of sense to me. Where as in reality, the real Big Boss has been tired of this shit a long time ago, Venom has the abilty to do something different. In the end, he too ends up becoming part of the horror, just like Big Boss. It's unavoidable, and Big Boss understands that now. None of this story even really needed to be told. We just wanted to see it. Venom gave us a game, where Big Boss didn't need to. And that to me makes much more sense.