After they took care of india and china they will not have 19 airunits left.
Taking india and taking care of china normaly involves losing air to take the countries, and india has 3 AA guns and the potential to build a pretty big stack of inf-art with it.
Also the investment of 8 carriers is a lot more then japan can build in a single round, it takes at least 2 rounds to build that amount, and US can just send in a fleet of subs and sink them all.
Japan will loose around 2 planes, on average, taking India. This is because Japan will have enough ground to stack Burma and prevent a UK attack against it, unless UK has put all of her planes in India, which would need to be around 8 at the very least. A well played Japan will have your fleet positioned off Japan/Philippines, so the US won’t be able to just kill that fleet. You don’t need to move your fleet off the coast of India to take India herself, maybe a couple fleet pieces can move there to protect your transports, if you need them in the assault. Besides it will take Japan a couple turns to regroup all of his units to strike Hawaii. After taking India, which normally happens around turn 7-9. Lets say it Japan takes India late, on turn 9, and then their planes are non combat moved to Yunnan. On the turn you take India you are building subs/DD’s. The following turn you build the carriers and the planes move to the Philippines/carriers you already have. Keep in mind that Japan should be be spending 70% of her income on fleet for most of the game anyways. Turn 11 you would put the planes on the carriers in Japan and set your fleet up for a strike on Hawaii, with Germany having 2 bombers to kill any blockers the allies might throw out. I have played many games against myself, and the only way to defend against this is to pull every US fleet piece, that isn’t a transport, from the Atlantic back to the pacific to stop Japan from winning.
Maybe you aren’t playing against a Japan opponent who has the foresight to know when they are going to hit India by turn three because of the layout of the allied units, and their own units. The hardest part for Japan is getting enough ground to stack Burma, but once this is accomplished India always falls with the loss of only ground, plus the planes killed by AA fire. Even in my Games where US goes 100% pacific Japan is still able to take India unless UK sacs Russia early by keeping their fighters in India instead of Moscow. I think where most Japan players go wrong is not being completely consolidated in their efforts to take India, maintain the DEI, and keep China at bay by keeping their forces grouped on/around Yunnan. The real reason, IMO, why Japan can win so easily is because German bombers can can open any blockers the allies put up making it a nightmare for the US/Anzac in the pacific. If US were allowed to stall for a turn by putting up a blocker that would be huge in allowing a little more leniency in the way the allies play the map. Sigh I really can’t stand how powerful bombers are on this map :(.