I bet there are CYA reasons that current smoke detectors are as annoying as they are. Smoke detector or not, it is inevitable that there is going to be a fire, and people will get hurt, and nobody wants to be the maker of a smoke detector that wasn't as annoying as possible about signaling for battery replacement.

With the nest and the text message, I would worry about the text number/account credentials getting stale/lost over time, and then when someone gets hurt in a fire there will be liability shenanigans.