Having to rewrite an entire struct to change one value within it seems... counterproductive, considering that the purpose of having structs is largely speed-based.
I can see it being useful to be able to flag a struct as immutable for certain cases, but for there not being a mutable version?
The main point of value types is that they're passed by value. As such, they have no concept of identity. The value 2 stored in variable x is the same as the value 2 stored in variable y. So, too, the value 2+3i, represented as the value type Complex { double r, i; }, is the same no matter where it is stored. Introducing mutation changes that. Values can now be destroyed. This is the conceptual reason.
The practical reason is that structs help you with speed when their size is no more than the size of the machine's cache line, i.e. up to 64 bytes. Beyond that, you incur an extra cache miss when accessing them (even below, if they're not aligned, but the smaller they are the smaller the chance), so you might as well use a pointer, and your copying costs are no longer negligible, so a pointer might be a better idea anyway.
So conceptually mutable "values" are a bad idea, and there's no reason to have them because beyond the cache line size they might cost you in performance more than they buy you, and below it mutation doesn't help as copying costs are very low.
The only benefit mutable structs might have is when you want to lay out a large number of objects, which you'll be accessing sequentially, consecutively in RAM. This way, the CPU's prefetcher will help you stream them into the cache. But the way HotSpot's allocator works, you're almost certain to get the same behavior with objects anyway, and if whatever tiny performance difference this might buy is that important to you, you might be better off using C, anyway.
The goal of the JVM is to get you the best performance for the least amount of work; not to let you work hard to squeeze out a few more performance improvement percentages. It sometimes certainly allows for that, but not at the cost of such a fundamental change, which is bound to do more harm than good.
