Supposedly 1 of the reasons why Namie decides to perform at Kouhaku for the final time is partially due to her son Haruto
In
http://tieba.baidu.com/p/5492927951 , it was mentioned that Haruto shown an interest in hip hop dancing & also plans to enter showbiz in the near future like his mum
Hence Namie decides to perform at Kouhaku which she hopes by doing so NHK will be able to take care of Haruto if he wants to participate in any programmes that NHK organised
Not 100% sure if it's true though