Since there are many answers already, I won’t elaborate too much. I just want to emphasize one thing: people mention immutable data structures as a default a lot, and sure, they’re great.

However, someone might turn around and say, “well, I can pull in a lib with immutable data structures in $lang if I want to.”

It needs to be highlighted that the real game changer is an /ecosystem of libraries/ built entirely out of immutable data structures.

That’s not something that can be engineered as needed.