Well engines are probably the most difficult part of an airplane to build. especially building a reliable engine. there are very few countries that have been able to successfully do it. you seem to be downplaying the indian problems.
their progress on airframes leaves a lot to be desired also.... I think Indians are a good example of what not to do.
also to say that going from prototype to production model is not a big deal/expensive is completely false.
just look at the f-35 production. Boeing was actually competing for and put out a prototype
https://en.wikipedia.org/wiki/Boeing_X-32 to try and win the trillion dollar contract. how much do you think they spent on it? not even in the same universe as the figures were looking at for the F35.
I really don't think you grasp how complex building a state of the art military fighter plane is. 1950s tech is not comparable to todays standards.
It is not a single machine. But many different machines and components that have to work perfectly together. That's the real challenge.
just to give you a single example. The trillion dollar f-35. After decades of work, production, the pedigree of a century of aircraft building and unlimited time and money going into it.
one of the many current problems it has is the "bump" when it takes off from aircraft carriers. There is a slight design flaw that gives the aircraft a "kick" when it takes off. And that is enough to sometimes even knock the all important helmet off the pilots head and temporarily disorient him.
They are seriously considering just leaving this design flaw as is as it would take many billions of dollars and probably years and a large re-design to fix..
you have to overcome many challenges like this from prototype, To production, to serial production before you get to the end product. to say that from prototype to finished product is a simple or cheap process is completely false.
that is probably what scared japan as well...
looking at the history of modern aircraft. virtually all of them have been greatly over budget, Late and faced with enormous difficulties.
If japan wanted to invest 40 billion in the program. They would have had to take the risks of massive cost overruns taking the project into 100 billion dollar categories with no guarantee of success......
their other option was to go for more of a sure thing. And join the American program. Which is the option they chose. There is room for debate on it, but to automatically dismiss it as some sort of national betrayal by Japanese politicans with 0 rational reasoning behind it would be false.