At least in my area precipitation is not predictable enough to rely on that way. We try to time our plantings with the rainy season but predicting which year will be wetter if dryer than average is more or less impossible. Last year was expected to be dry but it was very wet. This year is projected to be wet but so far has been dry. Long range models aren’t good enough to stake your project success on.
Plus, as you say, it would be difficult to maintain professional staff around working only once every few years. Such a system would be totally lacking in expertise and would also create much inferior working conditions. If the rains don’t come, you need to irrigate more. That’s just how it is and I don’t see a way around it.
I’m not sure I understand the breakdown of numbers there. Are you counting replants in your survival there? I would not include those because they are of a different age cohort unless planted shortly after the original trees. My number doesn’t include replacements.