The problem is not that the array is mutable, but that the size of the array may differ for different strings. (In fact, it usually* does differ if the string length is different).

This makes it impossible to flatten - as the VM needs to know the total size when creating the memory layout for a class...

* (with a small exception: if the strings use different coders, one can be twice as long and the backing arrays would still have the same size)

Indeed. At this point you’d need to bring the size of the string into the type system (C++ Templates we meet again!) but then you look at a solution worse than the problem…