Board Thread:Naruto Discussions/@comment-24757127-20140709042852/@comment-1512913-20140709100500

This. It's like absolutely unlikely that there are no Senju beside Tsunade but 65 years after the village's founding. There should be quite a few in fact. Nothing suggests that they've gone to the moon.

The most logical assumption in my opinion is that they simply ceased to be a military clan (hence ninja) and are now monks and so. Or they sell ramen.