Let me give it another try so that your tiny brain may understand. The last try though. So there are two large clans Senju and Uchiha who unite and form a village. In time the village prosper and other clans join in, the village grows bigger as more clans keep joining in. Now the uchiha clan maintain their presence through incestuous means mostly, while the senju are a more open society and do not mind mingling with other clans and progressing their clan.
Overtime the uchihas are still thriving by reproducing within their clan whereas the Senjus all loose their *** drive and eventually die off, all of them without furthering their clan....

Only an idiot would come to such a conclusion. Just because the manga didn't focus on any other senju doesn't mean they're extinct.