After all the drama that led them to this point, SMAP‘s final performance ended up being mega-hit “Sekai Ni Hitotsu Dake No Hana” on the final episode of ‘SMAPxSMAP‘.
https://www.youtube.com/watch?v=eoFvRmDA7wA
The song they choose for their last performance is the best-selling song since the turn of the millennia and the third-best selling song ever in Japan, and this article helps explain the meaning behind the song, why it became so beloved as an anthem, and ties it to the massive impact of SMAP on Japan.
Given that, I suppose it was the only way to bring SMAP to a close.