Hmmm.... Not sure why, but the calculations from the new SX1262 transmission current measurements(see picture below) indicate the 2xAA batteriess would last 38 years using SF8 and transmission power set to 16dBm, which is obviously less than what I was predicting. I haven't yet increased the number of pre-amble symbols, so the reason isn't that. This time I did power the radio module with 2.8v instead of the 3.0v I used in the earlier measurement, so maybe that had something to do with it. Not sure at this point. Go figure:
Anyhow, regardless of the reason, that's enough battery capacity headroom that I'm not worried.
[Edit: I found the reason: I had lately increased the coding rate from 4/5 to 4/8, which increased the airtime over what it had been previously at 4/5, but I had forgotten to include that change in the Semtech LoRa calculator calculation.
If I use the 47.232ms Time on Air number shown in the LoRa calculator results here and redo the previous lower bound calculation, the answer is a lower bound of 37.35 years, which is indeed less than the 38 years calculated based on the new measurements above. ]
[As an experiment, I tried powering the same SF8 mote with a solid 3.3v and a transmit power of 22dBm provided by 4xAA batteries and an LDO. The result was about 1% missing packets. Then, reverting to just 16dBm, it was a roughly 2% missing packet rate. Therefore, it's probably not worth the bother of powering the mote with 4xAA batteries and an LDO. Granted, it would be some improvement, but, IMHO, not really enough of an improvement to justify upgrading to a 3.3v supply source. ]