Even a smart compiler can't break the program semantics, and without a closed world assumption it simply can't assume that an entirely different part of the program doesn't expect to observe object identity for a type.

Java is adding small, orthogonal features that amount to the same feature set as value types in other languages, but can be cherry-picked into existing code for partial advantages without significant changes. These value classes are still nullable, lack a guaranteed layout, and can't be observed torn, unlike in C++/C#/Go.