What I was trying to say was that the battery dropoff is bad enough when the battery is fresh, but after 3000 charge cycles, the voltage drop off rate is significantly worse. Further, the fact that the voltage would drop below your 3.3V limit after just a few transmits (even with a fresh battery) at very cold temperatures, means that you will need to charge much more frequently than once a day, hence, battery life won't be anywhere close to the 8 year life you're calculating.
One thing I don't know is which radio you're using? Is it the RFM69W or the RFM69HW?
Tom