Why do you want to the Sage to be an uzumaki that bad? this is just fan fiction not fact, people. The main counter to this theory is the hyuga clan argument. Kakashi clearly stated early in the manga that hyuga clan and uchiha clan respective dojutsu seem to stem from one another, which technically means that the hyuga and uchiha are related, 'distant blood relatives' - and this also works for the senju and uzumaki clans. It is said that the senju clan originated from the younger son's bloodline,it didnt say that the son himself was senju and senju only, I will also apply this to the older son and you will get that the older son is not uchiha.
To make it simpler, its like the theory of evolution, we didnt evolve from apes BUT we have a common ancestor! Common ancestor, people, common ancestor.
And I think that with the sage's death and the quarrel of his two, the royal bloodline was no more.