Maybe I'm missing something, but how would GTA6 source leak really harm Rockstar? I mean it's unlikely it would be possible to compile a full working game from the leak, and even if so, it's such a non-trivial task, that I don't believe it would hurt sales /that/ much.
The only thing I can imagine is the story would get spoiled on the internet, but that's about it.
I feel the need to say it shouldn't be this way, to avoid an onslaught of replies, but:
It would be dramatically easier to discover and exploit vulnerabilities/glitches in their multiplayer experience, which is their cash cow.
On the other hand, maybe the community could submit bug fixes for loading times.
https://news.ycombinator.com/item?id=26296339
It'd make it a pain to stop abuse of their online platform when it launches, which is financially problematic given gta 5 online made rockstar billions.
I would speculate that it’s not about individuals compiling and playing without paying, but that with access to the codebase, creating cracks and online cheats would be trivial, which might actually hurt their bottom line