Plateau-Rayleigh instability─a macroscopic phenomenon describing the volume-constant breakup of one-dimensional continuous fluids─has now been widely observed in adatoms, liquids, polymers, and liquid metals. This instability enables controlled wetting-dewetting behavior at fluid-solid interfaces and, thereby, the self-limited patterning into ordered structures. However, it has yet to be observed in conventional inorganic solids, as the rigid lattices restrict their "fluidity". Here, we report the general fluid-like Plateau-Rayleigh instability of silver-based chalcogenide semiconductors featuring soft-lattice ionic crystals. It enables postsynthetic morphing from conformal core-shell nanowires to periodically coaxial ones. We reveal that such self-limited reconstruction is thermodynamically driven by the surface energy and interface energy and kinetically favored by the high ionic diffusion coefficients of subnanoscale soft-lattice shells. The resulting periodic heterostructures can be topotactically transformed for epitaxial combinations of functional semiconductors free from the lattice-matching rule. This fluid-like behavior in soft inorganic solids thus offers routes toward sophisticated nanostructures and controllable patterning at all-inorganic solid-solid interfaces.