[technomancy]

> you can get both in Lisp if you want

The whole point of benefiting from immutable data structures is their ubiquity. If they're something you reach for only when you personally decide you need them, they're unlikely to work well with 3rd-party code; you will have to do a lot more digging to understand how the code you're writing will behave in a concurrent context vs having it be obvious. Making it opt-in is almost making it pointless, unless you never use 3rd-party code.