Excellent Post.
But senju DNA is required.
Madara achieved the rinnegan AFTER fighting Hashirama. Yes a near death experience through all the stages of the sharingan adds weight to your claim that the Rinnegan is unlocked after a near death a$$ whooping, and upgrading to EMS requires you to murder your relative. It does lend to the notion of violence needed to progress, but Madara would HAVE had to have fought him to get any where near hashirama's DNA. Madara wouldnt be able to just saunter over and ask Hashirama for a pint of blood without getting something thrown at him, he'd have to forcibly take it from him. That's probably how the whole fight started.
The younger and older Sons were blessed with the body and the eyes of the sage respectively, Two parts of a whole. Uchihas are decedents with the eyes and Senjus are decedents with the body. Both Sons were incomplete, one wasn't more whole than the other. Of course the one with the body that has an aura will LOOK more complete than the one with only eyes.
Yes it's true that Nagato had Madara's eyes. Madara chose Nagato because He had a suitable body for the eyes, just the same as Kakashi with Obito's eye and Danzo with shisui's eye. I agree with you that the sharingan places huge strain on the user if they're not uchiha.
Kakashi used to get so drained using the sharingan, he's improving, but he wasnt as good as Danzo using it.
Danzo, he was already in possession of shisui's eye and could use it fine, but he got Hashirama's DNA for that extra boost, it wasnt NEEDED in order to use the sharingan but it helped.
Nagato had the body and lifeforce to support the Rinnegan, without the help of senju DNA because he is already a close decedent.
Danzo has a stronger body than Kakashi and Nagato has a stronger body than Danzo in addition to his bloodline senju DNA, that's why he's able to manage the Rinnegan. This was also done on purpose, i believe to demonstrate the notion that extremely powerful senju life-force is required to use the Rinnegan.
Had Danzo or Kakashi been given a Rinnegan with no senju DNA, i believe it would have reverted to a Sharingan, or at the very least an EMS if it didn't just wipe out all of their chakra.
To become complete (Rinnegan) an Uchiha needs the body of a Senju, again two halves of a whole. Its clear that by body, they mean an individual's LIFE FORCE which equates to an individual's DNA. Each individual's life force is different the same as each individual's DNA is different, but Clans share similar life force and DNA as they are relatives.
Madara could have taken the DNA from any Senju but why would Madara Uchiha, a blood thirsty war monger who seeks ultimate power, go for a weak senju?? He would want power from the strongest and most powerful senju. We all know that DNA is shared in clans, but there are individuals in these clans who's abilities manifest stronger than the other clan members because they have their own individual DNA. You wont get stronger stealing a weak person's DNA.
That being said, it again only makes sense that the strongest Uchiha would target the strongest Senju because logically the Senju with the strongest DNA will have the strongest Life Force and provide the strongest power-up.
He could have went after any ol' senju but it would have been dumb and hashirama would have killed him for it any ways. It makes sense he would go after the strongest of the bloodline.
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