The whole point of MGSV is that Venom accomplishes just as much if not more than John did and proves himself worthy of the title of Big Boss, even finally being called it by John's former closest friend
MG1 isn't just Solid being commanded by a random nobody with BB's face, and in fact I'd argue that Venom is a better BB than BB was. BB eventually used the title to identify himself as a weapon that lived for the exploitation of the battlefield. Venom used the title in connection with BB's legacy as a hero to have something to aspire to. ---
