[TurningCanadian]

It seems like they're worried that interning of objects/records would be too expensive to do generally. It's hard to predict though: that overhead would only apply to new code using R&T and has to be weighed against the elimination of recursion for deep comparisons and freezing, fewer re-renderings when value-equal but not reference-equal objects are encountered, the greater possibility of memory reuse across deep clones, and other performance optimizations that would be unlocked by true immutability.

Aside from performance, true native immutability would bring huge improvements to how JS programmers can reason about their code. Not having to worry about mutation makes a whole class of possible bugs disappear. Having to rely on third party libraries (or deep freezing manually) for immutability is really holding back the language.