OK, I hope you are correct- but I have spent quite a few years in that part of the world, and I do not see the Japanese people being supportive of such a use the JSDF.
Also, there is a political/historical problem, in that the Koreans have not forgotten completely the horrors of the long Japanese occupation. I wouldn't think that North or South would like to see Japanese soldiers there again.
You may well be right. I think we have to be careful about rearming Japan -- there are a few nutcases there still. But we need stronger allies in the Pacific.