Random, semi-related observation: the Nokia 5110 (released in 1998, 20 years ago) as THE phone of the era had 1-3 hours of talk-time, and 40 hours standby. Roughly the same as what the Apple Watch cellular gets right now: except this one will take your heart rate, gps track, and countless other functions. Not a bad start for the device.
(Most people who have an Apple Watch will tell you they easily can get 30-40 hours out of the battery - which does mean charging every night to guarantee that 2nd day. that 18-hours usage is low-balled)