The big-box retail model that turned Wal-Mart, Target and Kmart into colossal successes is now half a century old, and experts say there are no guarantees the big chains will be around in another 50 years. The rise of e-commerce means physical stores could evolve into showrooms rather than one-stop retail spaces, says business historian Marc Levinson. "[E]ven if they still exist, they may be quite different," he says.