[encloser]

> ...but then forget that they've now created a copy of their entire system

With persistent data structures you don't have a whole copy of the entire system. The two views of your system share memory where information didn't change. Sort of like how Git works.

[robbbbbb]

You're absolutely right, and persistent datastructures are exactly the right solution for this problem domain. But that doesn't get round the fact that they are a hell of a lot harder to implement and to understand in comparison to what you had before in an imperative, mutable world (as a UI programmer I now deal with diffing and versioning whereas before I simply mutated variables).