None of these countries are truly self-sufficient. Just look at the current difficulties between France and Germany regarding the sixth-generation fighter project. All of the countries mentioned rely on cooperation to varying degrees. There are plenty of examples, including Japan, which has increasingly pursued international partnerships in defense technology. And they certainly don’t do this out of affection for one another, but because the complexity, costs, and strategic risks of modern defense projects make cooperation unavoidable.
A good illustration of this is the GCAP (Global Combat Air Programme) involving the UK, Italy, and Japan. Similarly, South Korea has worked closely with the US and other partners in the development of advanced systems, while France, despite its ambitions for strategic autonomy, often depends on European partners for large-scale projects. The UK has also historically shared development and production responsibilities with allies, from joint missile systems with France to aircraft cooperation with the US.
As for Taiwan—how could it possibly be considered self-sufficient? While it does produce certain defense systems domestically, such as missiles and naval platforms, it remains heavily dependent on the United States for advanced fighter aircraft, missile defense systems, and critical technologies. Without external support, Taiwan would not be able to maintain a credible defense posture against the threats it faces.