Nah, there are already smart watches with both cell functionality and bluetooth (which is the only sensible way of using them as a phone) that easily lasts 2 days of regular use today. The 1,5 year old Samsung Galaxy Gear S being a good example, still my favorite in the smart watch category by a huge margin.
Bluetooth these days does not need much power. Back in the day people used to turn off bluetooth on their phones when not having a device connected to save power, I just leave mine on all the time now, doesn't seem to make much of a difference.
Somehow I don't believe you... I'm sure it would last 2 days if you don't actually do anything with the watch, you have just claimed a watch that is older than the Apple watch, lasts double the time an Apple watch does currently yet with more battery draining features and a 300 mAh battery.
What's the talktime of the watch?
I am currently using a Samsung Gear S with my Note 3. The battery on the Gear S goes for 2 days with moderate use on bluetooth. On cellular, probably less than a day.
Cellular connectivity had been a game changer for me on how I view smartwatches. My first watch was an LG G Watch. Great Android Wear watch but for all practical purposes, just a glorified notification device. Had to have a phone with me for the watch to be of use.
With the Gear S, I can forward my calls and texts to the watch and go out for a long walk without having to carry my phone with me. It's got GPS with offline maps if I ever needed them. And I pay $3 a month on a prepaid Tmobile plan for this convenience. I can go on and on, but suffice to say, in this age of huge phones, a cellular enabled smartwatch is a very useful accessory.