My estimate: The phone-enabled (LTE) Apple Watch is 2 – 3 years away from being an adequate smart phone replacement and 4 – 5 years away from being a which-one-should-I- choose? possible iPhone replacement. Two main current issues: (1) inadequate battery for anything but occasional cellular independence from your phone and (2) additional fee ($10) charged by cell companies, though your LTE watch uses the same phone number as your phone.
In 2 – 3 years, I expect that battery time will be adequate for extended periods away from your phone. In 4 – 5 years, I expect that cell companies will have to allow watch-only phone lines, and that people will begin to increasing ween from their phone and use their watch for communication (and likely further move from email to texting). That will also make tablets a necessary companion, but one that you won’t need to take with you everywhere.