[blagie]

I will ignore the potentially false statements and restrictions on arm sales, and answer the high-level question:

Force design is a question of what the force will be used for. A military for a counterinsurgency in Iraq, will look very different from one for a peer-level conflict between superpowers. A military designed for force projection will look very different than one used for domestic defence. Etc.

There is a fairly standard way of doing the analysis for what makes sense for a given country's specific defence needs and wants:

* Most of India's defence needs center around Pakistan, as well as skirmishes of various sorts. I can think of few things an F35 can do which India needs done which can't be accomplished by a much cheaper plane.

* Japan's primary goal issue is China's growing military base. I can think of few uses Japan would have for a much cheaper plane.

India's answer to all-out war with China are nukes. Aside from that, it has no chance for military parity in the next half-century or so.