I don’t understand this comment - the .NET CLR supports arbitrarily large value types. Are you referring to something like “atomic flattening of those types” instead? Because the CLR doesn’t guarantee that, and therefore supports flattening.
Basically JEP401 guarantees that just adding "value" to the class won't change behaviour, which also means that you are not allowed to see object tearing, which requires atomic operations on the field.
Relaxing that would break safe publication rules, at least for Java.
This is the correct default as the vast majority of developers using value types will not be aware of tearability, being taught that "value types are safe for parallel programming" without knowing nuances.
> For the primitive types longer than 32 bits (long and double), it is not guaranteed that reads and writes from different threads (without suitable coordination) are atomic with respect to each other. The result is that, if accessed under data race, a long or double field or array component can be seen to “tear”, where a read might see the low 32 bits of one write, and the high 32 bits of another. (Declaring the containing field volatile is sufficient to restore atomicity, as is properly coordinating with locks or other concurrency control.)
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[0] - https://openjdk.org/projects/valhalla/design-notes/state-of-...