I was about to cautiously criticise the Japanese decision as impatient, though I was sure they had good reasons so this is very helpful Maruben. Thank you!

You are welcome.

 I explained in the first post was more or less the official  reasons to have additional F-2:

1. China has increase its number of fighters (as usual).

2. The Korean Peninsula is tense again (or as usual).

3. The F-35 is delayed (nothing new).

4. There will not be more local production fighters from the years 2012.

Number 1, 2, 3 are well-known fact  for several years. 

Therefore, I think number 4 is the most important factor since we need to keep our basic and minimal manufacturing skill and staff in the business as long as there is no other fighters to build locally in the close future, unless the Pentagon uses its muscle to accommodate the construction of any part or/and assemble of the 50 F-35s the Japan wants in Japan. 

It is my opinion that the Pentagon, Lockheed-Martin and Japan had agreed on that, it is to allow some production of part or assembling of F-35 in Japan, however that has not made public. Especially because the money needed for the purchase of the F-35 is already allocated in the next fiscal year budget just waiting to see and confirm that the F-35 is as good as Lockheed-Martin is telling us. 

It seems that allow Japan to produce some part or assemble of the F-35 locally is a breach of the JSF's program, but to allow the Japanese military industries to keep its capabilities in the fifth generation fighter is an important part of the alliance US-Japan as the SM-3 program has proved for anti-missile defense.

The original reason why Japan is not in the JSF was because Japan had a prohibition to export weapons to any country except the US. Since this prohibition has been repelled just few day ago in a very timely fashion just to facilitate the export of the SM-3 to Europe with its Japanese made parts, there is not now any legal restriction to be out of the JSF's program.