That would be unfortunate. The battery in my key fob lasts for years.
Perhaps that is the answer to the dead battery issue. When the battery is depleted past a certain level, it switches to a mode where only certain essential functions are available. If my car's key fob can last for years on one battery, couldn't the Apple Watch's battery provide enough power to emulate the key fob for a day or two after it's too low to do all the other stuff?
IF you don't mind the screen not working (what uses the most power by far), probably can last a good deal more. Wonder if they could have a voice mode, with no screen at all.