There's no breakthrough necessary. The batteries are already good enough to manage standby with an occasional call, which meets the _phone_ needs of most users. The Apple Watch today is at least as good as a phone today as the iPhone was a decade ago. I can't even imagine the complexities of carrier/Apple politics to know what the real reasons are for why Apple hasn't allowed the watch to be standalone, but they aren't technical.

Which Apple Watch are you using? Because I can't make it a full day cellular only. It will die. I've tried. I have the series 6 with latest WatchOS.

SE. How long is a day for you and what are you doing on it? Mine lasts at least 8 hours with light use (mostly standby with an hour of music but no streaming) before I decide to charge it around 50%, which is about what I'd expect of my phone 5 years ago.

Are you using cellular exclusively during those 8 hours? "Apple Watch Series 6 (GPS + Cellular) usage includes a total of 4 hours of LTE connection" according to Apple. (https://www.apple.com/watch/battery/).

My LTE only usage includes about 30 minutes of streaming and the rest just glancing. I can't get through a day like this.