Pure functions excel at reducing bugs, I'll give you that. They also excel at transforming state. But managing it? Encapsulating it? No thank you. I'll take plain old classes over every state management idea that has ever been conceived for Haskell. Does anybody seriously believe they will manage the state for hundreds of thousands of different sprites, environments, bots, and players using the State Monad? I'd rather castrate myself. It's one thing to take immutable/functional ideas and code and use it in a stateful system, and another thing entirely to constrain 100% of your code so that it all fits in that neat theoretical box you've built for yourself.