We are at a peak right now with batteries and their make up, until someone changes the material inside to add more capacity in a smaller package.
Of what I am aware of, we are still 5 to 10 years out from better battery make up and capacity that will be available to the market.
That is what the main problem is. A phone is a device you expect to charge every day or other day, it's just how it is, or always has been. Battery tech has not changed much in this aspect, they might of gotten a little smaller over the years but, for the power output, very minor. As far as I know (I don't clam to be an expert in this area but, I know a enough to make this statement), we are using the same basic tech in batteries that we were almost 10 years ago. 5-10 years ? This should of been here years ago.
Now using this SAME tech in battery, your dropping into a watch. with the stuff needed to make it work, so it's otherwise really bulky, or it's got a hit on the battery where the device needs to strip features to meet a reasonable battery life.
For MANY years (from my first Moto Star-tac), I expected one thing, to charge a phone every day, or other day. My watch ? Lasts 6 months to a year on a single battery (standard analog watch). So moving to a smartwatch, pulling it off every day to charge every day, it's not something I like and it's another thing to worry about.
If a smarwatch can last a week or so, it is things I have to worry about less, in about 5-6 days on my pebble, I will get a 20% notification, and if I let it go for another 2 days or so, it will give me the 10% notification. Not a pest but, a simple reminder.
All in all, to each their own, If you don't mind charging a WATCH every day, then cool. I just don't like it and will avoid it as much as possible. I pretty much only take the watch off to take a shower (and it's water proof so I don't need to do that if I wanted to), so it's another factor.
Tech moves forward at a crazy pace, faster better is always coming out almost every week but, when it comes down to it the ONLY tech that does not change is batteries, and we are in the dark ages in this aspect.