Re: Smart watches should be simple
For most use cases, you will not be fine with a weekend's battery life. For one thing, anything that builds a battery with the goal of it lasting a weekend will have a battery that really works for twenty hours. If you're going to build a battery for a weekend, don't accept it until it lasts a week in testing. Some watches are capable of tracking sleep schedules, so you don't charge it at night. Useless feature? Fine. How about that watches can use their vibration feature as an alarm. A silent one that doesn't wake up people who sleep nearby, especially if you're a sound sleeper. Incidentally, if you like that idea, I recommend the Xiaomi MiBand 2 (max $20) with the gadgetbridge app from fdroid.
There are a lot of convenient things a smartwatch could do that would reasonably use up battery and make a month-long life untenable. However, if your users also have a smartphone that does a lot of that, there has to be some discussion as to what things the watch can leave to the phone in the interest of having a good battery life and a good set of features. Telling the time is critical. Most people who buy these watches want notifications. Do they use the watch apps? Do they reply to messages with the watch? Do they really need the watch to connect to cell towers, or would having a WiFi chip that is turned off unless requested be good enough for emergencies like my phone died?
It's not just cost, although that's a problem too, but the fact that the more features nobody wants are shoved into the thing, the slower it runs and the faster it dies. You probably wouldn't buy a laptop that I built where it has three hours battery life, but it also has processors of many different types so you can look at how your code runs on each different platform without leaving the interface. And also it has two independent bluetooth interfaces, making it possible to connect to more devices and turn your computer into a bluetooth peripheral that still has connectivity as a host itself. These features, while you could probably think of a way to use them, aren't in demand. The cost of them, less battery life and a more expensive computer, don't justify it for you.