1. You keep saying all evidence points to Tsunade being the Last Senju. Where exactly did you find Tsunade being the last Senju. Tell me your source. Then, i'll agree with you.All evidence points to Tsunade being the Last Senju. Fact Japanese culture like western Culture The Male members Pass the family name to their children's, Females become part of Husband Family. A Senju Woman marring a non Senju Her Children have the name of the father family. 2. Over one hundred years of fighting to establish and maintain the village a Senju had to take the lead and be the example for all to follow,that means the first in a fight was a Senju and the last in leaving the fight. Thats the Prize of Leadership. People die and levee no descendants thats a fact of life. Tsunade's brother was a kid when He died. How many Senju like Him died in battle as kids never to know a woman.
2. A senju child born to non-senju mother but senju father is still a senju. So like i said before, Minato Namikaze's mother is definitely from Namikaze clan, and there is still a chance that Minato's father was senju(however i do not say Minato was 100% senju, its unsolved yet).