You are correct, thats easy to reason about unless you are writing the garbage collector, in which case its pointless to write a garbage collector because there will never be anything to collect and memory will expand infinitely.

As for the persistent data structure comments, that has nothing to do with the fact that every function in a functional program has to copy data at such a ridiculously fined grained level that all you are doing is writing a garbage creator.

This data that every function creates in the name of immutability is meaningless, void of any purpose and is a bug.