Why not just give apps chunks of non-contiguous memory and let them deal with that fact? Then like realloc, the memory they need stretched may just be moved first.
But why would we do that when the hardware already has everything that's needed to present the app with a flat contiguous memory space? Why push unnecessary complexity at every single wasm app out there?
Besides, why would you design around the lack of a MMU? Even in embedded space it's often there.