To be honest this doesn't make any sense. SO6P had rinnegan. He didn't kill anyone to obtain MS. Same goes for Elder son of Sage if the doujutsu he had was rinnegan.
They didn't took their brothers eyes to gain EMS. I don't see how rinnegan could have fallen so low in years. There has to be more to it. We need an explanation before it makes sense. There has to be explanation behind these retarded rituals.
To me it makes some sense.
I don't think we'd all be interested in the manga if all Uchiha's had Rinnegan?
Speculation/assumptions
The Sage's sons, that had the eye (Rinnegan) it was obviously not on par with the Sage's Rinnegan as he had not only the eye but the body (thats why the Sage made sure none of them would abuse their powers by giving them his complete power).
The Sage's son, when he created kids they probably had Sharingans (It's a cycle SO6P Rinnegan, his son's Rinnegan, Sharingan, and than Bakyugan), not Rinnegan and as the cycle went they were all born with Sharingans, if Uchiha's had Rinnegan's, two of them can ran an entire village, which would be stupid. This is where Kishi brings out that the EMS can turn into the Rinnegan. As you can see Rinnegan isn't easily awakened reason why I think Tobi started war, as we know the Rinnegan is awakened in the moment of chaos, but also to gain the SO6P powers you need Uchiha DNA, EMS, Senju DNA (maybe the body too?), Rinnegan, Juubi, etc.
The Senju powers (SO6P body) I'm guessing is the cloak Naruto has, which is quiet good, and I'm sure it will get only better, as where Uchiha EMS can evolve to Rinnegan which has a long process you go through.