No Assumption they couldn't if they could at the time they would have continue with their own path instead of relying on US this is the truth no shame in it, not forefront of technological frontier when it comes to Aerospace/Aviation post WWII fighter development Name me one serious Fighter Jet Air Superiority Multirole solely development without cooperation of US. Yes but now they are serious and lets see what turns out without US which is highly unlikely US would be involved at some point...Almost all equipment you mentioned have roots of US technologies in them no surprise...by the way F-2 the F-16 spin off is an extremely expensive aircraft one can guess FX- project would be twice expensive and limited in Numbers just as F-2 merely between 82-89 infact it costs 35% more than F-16 Block 60/61.
You have very valid points @Luftwaffe. Originally, Japan wanted to purchase F-22 Raptors , but a US Congressional decision prevented the sale. This was the very reason, the catalyst for Japan to procure its own design. No doubt that we will take into consideration technologies we have learned from the United States. Mitsubishi Heavy Industries as well as Ishikawajima Harima Heavy Industries have a long history in working with the American defense firm Lockheed Martin, and Boeing in regards to airframe, and avionics. So the lessons / information acquired during their past co-operatives have been implemented.
Lastly, I'm not going to argue on the American military defense capability and design processes. They are, objectively, leaders in this field. So, our ability to tap into said R&D serves our interests. As I'm sure you will agree with me that there are many nations around the world that could wish to have direct access that we have. But are denied so.
Best,
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