The question is: how long are we willing to extend the set of primitive types? If we have u512 without hardware support, why should we stop there? Then why not bake in u1024, u2048, and so on?
I think at this point it's unreasonble to expect extending the set of primitive integers even further ad infinitum. There should instead be a single, const generic, preferably non-heap-allocating, arbitrary-width integer type for those who want to do big math. It could use intrinsics to lower to LLVM's arbitrary-width integers as well. This would resolve the endless debates as to what does and doesn't belong in the core language.