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That's pretty much true for real-time games. There are turn-based games that can be played on a company-provided server, and some extreme cases where games are actually decentralised (like blockchain chess). The open source RTS 0ad used to have every client compute every game state, but I believe that's no longer the case. Without smart contracts you fundamentally can still have decentralised computation of game state, and even preserve some asymmetrical knowledge by having players encrypt orders and share keys after all players have submitted orders. A decentralised game of Diplomacy could be implemented this way. Have each player submit a seed and use the combined seed as the basis for a pseudorandom number, and you could implement something like Civilization in a trustworthy manner without a central server of any sort.