Board Thread:Theories and Speculation/@comment-26904721-20150820181613/@comment-734582-20150821115316

BakumatsuWarrior wrote: but by the end of Part I he had broadly sketched the overall future plot in his mind. Hence the statues of Hashirama and Madara at VotE at the end of Part 1. During Hiruzen's flashback to many Konoha Ninja, a team photo of kakashi, rin and Obito (with all characteristic features) can be seen. just wanted to add that.

Considering that the Senju were both under Hashirama and Tobirama (who believed in the Village above a clan), and that they were the strongest ninja alongside the Uchiha, the answer is likely that they died out through battle and marriage. Unlike the Uchiha and Hyuga clans, who specifically upheld their Clan status, many other clans just integrated. Hence, the Senju clan likely got dissolved.