I would bet the reasoning behind dropping the anti-aircraft sights on the Type 99 was that;
1) Making and installing them wastes time and materials
2) The lead required for late war aircraft (400 plus MPH) would have made for a rediculously long sight
3) Under combat conditions the sights weren't used anyway, so what's the point?
The P-40 was a well armored aircraft with self-sealing tanks. A P-40 returning from an straffing run full of holes wasn't uncommon. That being said, how many didn't make it home after taking a round or two into something vital?
Is rifle fire ineffective against an aircraft, yeah. Sometimes though, you get lucky.