Different body compositions may allow for Nebula to survive the harsh conditions of the ship better than Tony, and maybe she’ll work out a way to save his life.
Nebula and Tony sharing a ship to escape Titan is probable, especially given they’re hanging out together shortly after Thanos’ snap.
Or she might just give him a heroic sendoff, as that chest touching doesn’t seem like the kind of touch a stranger delivers to someone who is gonna survive.