Why would it replace the watch? Phones and watches are either two entirely separate things, or indeed, once miniaturization and battery tech improve sufficient

Why would it replace the watch? Phones and watches are either two entirely separate things, or indeed, once miniaturization and battery tech improve sufficiently, the watch, being much more convenient to carry around and being instantly to-hand, glanceable etc, will replace the phone for everything except large-format display, just as with pocket-watches.