@kdfsjljklgjfg:
Let’s assume that Japan is making 40 ipc’s at this point in a hypothetical game. They got jet fighters last turn, their first try at tech. They buy 2 fighters this turn. Their net gain is -3 ipc’s, because they get two 4 attackers with 2 fighters instead of 1 tactical and 1 fighter. So they spent 5 to get the tech, and saved 2 by getting another fighter instead of a tac.
So even assuming they get 2 fighters every turn, it takes them 3 turns to actually benefit from that economically. Your point isn’t all that big of a deal, because you need to buy A TON of planes for it to make a difference that you don’t need tacs for the 4 attack.
You forgot to count in all the existing fighters (in japan’s case this would make a huge difference, as they already have a ton of planes!)
Also, fighters defend better then tacs, so there is an extra benefit (save 1 IPC and have 1 defence point extra. And a 4 attack even when going solo)