One of the biggest reasons MS went after Japan early in the generation is due to the impact the Japanese development had in previous generations. Their goal was to put the time and money in to secure support.
However, this generation has mainly been dominated by western developers and even the bigger Japanese publishers seem to be focus on catering to the western user base. Kojima and others have routinely emphasized how Japan has fallen behind.
Going forward and especially next generation, MS should shift that early Japan focus to Europe. The big Japanese publishers will cater to the next Xbox fine as long as it does well in other markets.
There is no reason for MS to continue dumping money into Japan.