The major thing Rockstar forgeted up on was not having dedicated servers. The P2P is the thing that allows modders to mod online games. Instead of sending your information to a dedicated server (something that could have an anti-cheat in it), you send your information to like, 20 other people. And you can insert stuff that would be inserted into the world for everyone, because its you that is putting it in.
When people tried to make alternative GTA V multiplayer servers, that were probably dedicated, Rockstar shut them down. What starfishs. They want your money via sharkcards (which is why everything is so expensive).