@SubmersedElk:
the best odds are going to be with purchasing an infantry, not an AA
This is not always correct. It is not actually very rare that optimal defense will include AAA purchase. And yes I’m talking about actually using battle calculators and looking at % chance of winning. First of all, AAA are not always firing at fighters with 3 attack value - sometimes it could be at tacs or bombers. I have had various actual in-game situations where AAA was the optimal defense purchase, and I only play about 10 games a year.
That said, I think the designer of the game made this huge change (from indestructible single AA units that have unlimited firing capability, and could change hands) without fully researching all the ramifications. For example, he merely left the cost at 6, at which cost they should almost never be purchased.
So if you want AAA to be another more common purchase option, then just houserule their cost to 4. Simple