Well they don't know that the Senju are nearly extinct, so I guess they just assume its the same as in their time. Either that, or whatever happened to the Senju already occurred back in their days.
Just like if any clan were to be gone besides a genocide. It would be the men that died in battle. While the women marry into different clans. They prolly were considered extinct in the middle of Hiruzen's reign as hokage.