Here's something to be taken from every piece of literature, yes, including manga. Just because things aren't said directly doesn't mean it isn't true or that or potentially true. When you look and notice patterns in things you'll understand that.
Using my own words, but notice I used the word patterns, which means reoccurance, which is probable cause for speculation/theories. What pattens have risen that would suggest that the Sharingan and Uchiha don't need the influence to unlock the powers that the Sage had?
How many times have Senju DNA and Uchiha DNA been linked together to provide an extrodinary outcome that has somehow been linked back to the Sage? When you have reason to believe something because of patterns, not just the words of another character that only gives a self imposed implication on the character's part (meaning it was inserted by Kishi for the character not neccessarily a doctrine of truth).
And this is where i disagree.
I personally have a theory that the elder son's eye was better than the rinnegan..
Rinnegan being what nagato had, What the elder son had would be something of the sorts of what madara has now, rinnegan with all the ms jutsu...
No where was it said that ms justu came from the so6p..
infact the red eye , aka sharingan, came from the JUUBI , and throught the juubi influence the elder son's eye turned into a mutated rinnegan.. a better rinnegan, Thus the uchiha clan started... with potential to unlock mangekyou sharingan, then rinnegan ms , which is what madara has,
I don't believe senju dna is needed for the evolution , you jsut have to be extremely special.. and in the history of the uchiha, sasukes eyes has been hyped to be special... by tobi, nagato and kyuubi even saying its more sinister than madara's ....
That all sounds well and good, and when I first saw the Juubi's eye I was wondering similar things but then again I was thinking about how the eye was a combination of what looked like a Sharingan and a Rinningan. I was thinking about how the Sage's eyes resembeled what looked like half of the Juubi's eye, and the Sage supposedly used the Rinningan to defeat the Juubi.
I developed my own theory that may be a counter theory to yours. I'm still on the whole, split down the middle gene thing between the Eldest sone an Youngest son. I feel that part of the eldest son's inheritance was the Juubi's power (the tome and Sharingan) which is part of your theory, however, when you look at the Sharingan compared to the Rinningan it's always a little bit weaker. We have no way of telling if the son's eyes were stronger then his father's and I'm one to keep the Sage at a higher esteem so what I theorize is similar to yours that they can achieve the Juubi's eye or whatever, which is a combination however the eldest son didn't get the full natural package to evolve into the Rinningan, which would be the Sage's power.
What I'm saying is the eldest son got the half of the full potential in terms of the eye, which would be the Sharingan techniques but it wasn't better then the Sage's origional eye, he only recieved the half that was the influence of the Juubi, that would also explain the hatred within the son as the Juubi is hatred and looking at Naruto's whiskers as examples, traits of a Bijuu can be passed down to offsping if the parent is a Jinchuuriki. I feel that you're right, that with the Senju DNA, they can achieve Rinningan and still maintain the Sharingan techniques. However I don't think it's going to grant them much more then that. Obviously the techniques of a kekkei genkai won't leave if you still have the genetic make up. It may sound like I'm changing what I'm saying and agreeing with your theory but what I'm saying is.
Once they get Senju DNA Uchiha just get the benifit of having both Sharingan and Rinningan techniques by the switching back and forth between the techniques, they're seprate entities so therefore the Rinningan still requires teh Senju half of DNA to activate the Rinningan for the Uchiha.
Am I making sense, because if I'm not forgive me, it's been a tireing day for mexd.