They're really sure that obito is 100 % correct, whereas the reality is, out of his 10 opinion/statements, 8o% of that are lies.
The theory or obito's so called ''the sons of the sage'' would become a fact once madara/hashirama or the kyubi confirm that with their knowledge. And I think kurama will be the one who can give us the real truth/story about that one. After all, he knew directly the entire story of the rikudou.
Also, I think anything's possible... someone mentioned - who's to say the sage only had two sons after-all? All this time we sort of thought Madara had one brother, Hashirama only had one brother. Now we know they had MORE than one. And perhaps that brother could have been an Uzumaki, who inherited something else that maybe we don't know about. 'distant cousins' can mean just about anything. If the Uzumaki's are an off-shoot of the senju, so be it... doesn't make them any less a descendant of the sage of six paths.
The Uzumaki name and lineage has been a mystery forever in this manga and only in the mid 300's or so did we start to get confirmation about Naruto's mother, then about Mito, then about the village being destroyed, etc. etc. The destruction of the village and 'why' is still an issue IMO, and looking at the seals they could do, and maybe better yet, UNDO, makes them something of a mystical group that seem clearly related. They were scattered about, Nagato was used by the real Madara, etc. Undoing the death God... with masks... cheesy, maybe, but I can see why someone would want them wiped out.
Anyway, if there was a third brother, it surely would have been mentioned by now, especially since the inheritances from the Rikudou Sennin is such a major plot point. Plus as mentioned, if they were connected in that way, then they wouldn't have been mentioned as only being related to the Senju clan, but also the Uchiha clan too. No one disagrees they aren't descendants, just that they aren't the main descendants people are trying to make them out to be.
To be fair, in-story, there wouldn't have had to be some mystical powerful reason for them to get wiped out. Since the Wave Arc, we've had the theme of those with unique powers being feared and hated. A far bigger mystery at this point is the Senju clan, and why they're nowhere to be seen or hear.
1. Most people considered Ridoku to be a legend..kakashi mentioned it somewhere...so not comparing to uzumaki is evident
2. Somehow i believe that mokuton is not a senju kekkei genkai, if that would had been the case, we would have had a few more senju having mokuton.....how hashi has it is a mystery and will remain a mystery
It would be interesting to see about s06p, if kishi actually shows us
It needs to be noted that not all of the Senju clan will be like the younger son, just like not all the Uchiha will be like the older son.
The older son with the powerful eyes and strong chakra describes Uchihas, but only a handful. Thus far only Sasuke and Madara fullfill both the strong eyes and strong chakra. Hashirama is the only one to have the strong body and life force from the Senju.
If we go by the Uzumaki's, Naruto is the only one with a strong body and life force. Nagato and Kushina had strong life force, but not the strong body (Sage Mode as in the case of Naruto and Hashirama). And lets not even get started on Karin.
If the uzumaki possess a very strong forces of life, then it means they got that trait for being a direct descendant of the younger son. Thats why they can live long enough. Whereas the senju was never noted to have that kind of traits, with the exceptions of hashirama and tobirama, I guess.
Does the uzumaki has large amount of chakra? So for having a powerful life force and large amount of chakra, then they're somehow more close for being a direct descendant of the younger son than the senju.
And what's wrong with Karin anyway? She got an amazing life force and strong body I think. yea, after all, her strong body/uzumaki trait made her to withstand the effect of being beaten of her entire body by anyone who wants to replenish/refuel their chakra.