FWIW I came to the comments because I also found the “Immutability changes everything” paper nearly unreadable.
I just wanted to comment that I love your last paragraph from a conceptual level. You want immutability? Great, let's talk about what performant game design looks like in that context. Games are special, in that they kind of do everything that computers are good at, to varying degrees. The author's ideas about snapshots of relational databases inviting further schema changes in the pipeline—I guess I see the idea you're trying to introduce, but what does it actually look like in a chess program? Or a choose your own adventure game?
I just wanted to comment that I love your last paragraph from a conceptual level. You want immutability? Great, let's talk about what performant game design looks like in that context. Games are special, in that they kind of do everything that computers are good at, to varying degrees. The author's ideas about snapshots of relational databases inviting further schema changes in the pipeline—I guess I see the idea you're trying to introduce, but what does it actually look like in a chess program? Or a choose your own adventure game?