If it comes down to immutability and copies being a bottleneck, a solution is probably to move more of the core game state into native code (like with Zig), but

If it comes down to immutability and copies being a bottleneck, a solution is probably to move more of the core game state into native code (like with Zig), but fault tolerance, scalability, and concurrency seem like important things for a MMO. Hot loading is pretty nice too.