Senjus never inherited the younger son's power. uchihas inherited the older son's "eyes" because they stayed breeding within themselves. but senjus were different peoples mixed together under one leader. all we know about them is that they were the strongest clan before the era of ninja villages and the only ones who could take on uchihas and live. but this is because they were basically jacks of all trades not because they had the "body"/chakra levels of younger son. his descendants likely branched into senjus and uzumakis and aburames etc. whereas elder son being more selfish made his sons keep the bloodline within their family (yay for wincest). the only one of real special interest in chakra levels and focusing on particular styles of fighting was hashirama. he got born with some random bloodline and that made him suddenly the best. everyone else i doubt had any kekkei genkai that special. at best there were probably a few ice users or lava users.
"senju" literally means clan of a thousand skills. it DOESNT mean "clan that has alot of chakra and strong physical bodies". such a trait probably disappeared as the younger brother's descendants married inot other clans for love or politics.