I agree with this, it makes the whole thing so dumb and without focus. I'm sure this has been mentioned quite a lot already, but it's also ridiculous how, in a series that started about cloning and which went on to talk about shaping people via information control, no one ever managed to make identical copies of another person. The Les Enfants Terribles were never exactly like Big Boss, nor was Raiden just like Solid Snake despite The Patriots' attempts to shape him in his image. Even in the king of retcons, MGS4, Big Boss straight up says that despite The Patriot's advances in genetic and information control, they "never managed to control people at will, let alone turn one person totally into another".
...
"Except, you know, that one time when they actually turned someone into an exact replica of me, 30 years ago!"
So, yeah. I have an extremely difficult time believing that Kojima ever came up with Venom Snake as a means to explain how "anyone can become Big Boss". It literally goes against everything that the series has been saying until now.
To me, MGSV's plot twist is a gratuitous fourth wall breaker with no consideration whatsoever for its implications within the narrative of the series at large, and also a weak ass attempt at replicating the plot twist that got most people to see Kojima as a masterful troll, that is, Raiden's twist in MGS2. Except that, as others have already said throughout the thread, this plot twist has nothing of the genius of MGS2's.