Do we not have GTA5 source already?
If 6 leaks before release, though, that's a completely different story. I can imagine them actually paying a ransom if that happened.
The only thing I can imagine is the story would get spoiled on the internet, but that's about it.
It would be dramatically easier to discover and exploit vulnerabilities/glitches in their multiplayer experience, which is their cash cow.
The failure is that loading times had been a complaint for years, and nobody involved lifted a finger. It would be impossible to use the platform without feeling the pain.
The software was released on 7 platforms, not counting multiple Windows versions. I don't know the risks or what platforms changes impact today or the test effort involved. I expect "it's still functioning as expected" was the default.
This is the sort of challenge hackers love, and the prestige is enormous. If it's possible with the leak, I have to imagine some group will do it.
Different era, but shades of the Half Life 2 leak.