Venom snake as a character tells a decent bit by proxy about big boss imo. You design him, give him your name, birthday. Venom was a completely different person and character before ttp. Considered one of John's best soilders within this cult of an operation.
Whats his thanks? Your thanks as the player? Becoming him. Memories replaced, face reconstructed, and made to stand in for him so he can achieve his greater plan. Becoming the biggest tool of them all. Being a proxy of him. An extension. An extension to him and his legend. Given a measely thanks over a tape. Then sent to die in his place.
All for some elusive greater good for soilders. A bigger picture you'll never get to see.
That is your reward.
Now some people argue that at the end, venom clearly is on board with it even after finding out the truth, yet I'd argue that even if he is, that's because of the cult like mentality of msf. Like yeah to any soilder on that base essentially becoming big boss and playing a big role in his plan would probably seem like a dream
