Are you sure it wouldn't be the opposite here?
Usually a game would have you play a 1-3 hour linear section and then transition into an open world instead of the opposite.
Wake up in a hospital, go through learning how to do all your basic tutorial like things like crawling, using melee, and finding your first gun, introduce various characters, and then land in the open world after escaping.
MGS5 isn't going to be set in a hospital. The entire thing is an elaborate troll.