I actually really like recur because it signifies intent. It tells the future-me that I intended for this function to be tail recursive (ie, it not being tail-recursive should be considered an error).

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.

(0) The compiler can automatically perform optimizations like fusing small nodes and hash consing.

(1) The garbage collector can take advantage of the fact that traversing (the immutable portion of) the heap sequentially produces a topological sorting of the object graph.