The wristwatch is jewelry. Its primary purposes are adornment and display of wealth.
Your gadget just has to be cooler and more ostentatious than one of my present wristwatches. It's not about being useful. You don't have to do a better job of telling time, or run applications, or try to supplant my phone.
Agreed, watches are no longer for keeping time. Phones have long taken over that role.
In fact, my watch still gets worn after the battery died. (Hard to explain, but the OLED time shows up after a button press http://www.google.com/images?q=DZ7080)
Your gadget just has to be cooler and more ostentatious than one of my present wristwatches. It's not about being useful. You don't have to do a better job of telling time, or run applications, or try to supplant my phone.