Board Thread:Theories and Speculation/@comment-26904721-20150820181613/@comment-26773508-20150821110114

Most probably they inter-married with other clans and gradually the Senju name disappeared. Although there could still be a whole bunch of Konoha nins with the surname "Senju" that we have not seen yet. Case in point: the Sarutobi clan. The only members who were shown throughout most of the series were Hiruzen, Asuma and Konohamaru. It was only in after the revival of the Juubi that we saw a whole bunch of Sarutobi clan members. It seems that Kishi didn't really plan on the whole "Senju vs. Uchiha" thing at the beginning of the series, seeing as how the so-called "Four Noble Clans of Konoha" didn't include the Senju, while the Uchiha and the Hyuga were included. Also Kishi initially wanted Akatsuki to be a bunch of demons instead of S-class criminals. Thankfully his editors turned down that idea and we got the Akatsuki that we know and love. Like the Akatsuki, Kishi may have had a different plan for Naruto initially, 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.