Isn't this attacking a strawman? Can't you always make object mutations construct new objects and return those? Basically "fake" the immutable part of functional programming?
Immutability is one aspect of the family of techniques called "functional". Yes, you can right immutable code in C or Java, but the compiler can't verify that intent in general, which means that a whole class of bugs are possible, and certain kinds of high-level general functions (combinators) and automatic optimizations (inlining and rewrites and skipping computations) are not possible to write/apply.
