The Rikudou Sennin devided his power to his two sons. The older brother has got the Sennin's eyes (Sharingan) and the younger the Sennin's body.
I don't think the Rinnegan is 'just' an upgrade of the MK. Its the combination of the body en the eyes, what can awaken the Rinnegan.
You need a lot of energy to control the sharingan properly, and even a lot more energy to controll the Mangekyo Sharingan. Therefore, Kakashi (low stamina) can’t use MS very well.
It would be possible, that the Rinnegan needs THAT much energy, that an Uchiha can’t even awaken it.
But what if you have a Senju body(or even Uzumaki; related to the Senju) AND the Uchiha eyes?
A senju(or Uzumaki) with implanted Uchiha eyes gets Rinnegan
Example: Nagato
- Maybe Tobi gave Nagato a Sharingan because he’s got the sage’s body, and therefore would get the Rinnegan(right away). Right now, Tobi got the rinnegan back, and uses it himself.
An Uchiha with implanted Senju DNA gets Rinnegan.
Example: Tobi and Madara
- After Madara fought with Hashirama, he got two things.
1. Hashirama’s DNA
2. The Rinnegan
He only fought to obtain the DNA. After he had the DNA, he also had the Rinnegan.
I don't think the Rinnegan is 'just' an upgrade of the MK. Its the combination of the body en the eyes, what can awaken the Rinnegan.
You need a lot of energy to control the sharingan properly, and even a lot more energy to controll the Mangekyo Sharingan. Therefore, Kakashi (low stamina) can’t use MS very well.
It would be possible, that the Rinnegan needs THAT much energy, that an Uchiha can’t even awaken it.
But what if you have a Senju body(or even Uzumaki; related to the Senju) AND the Uchiha eyes?
A senju(or Uzumaki) with implanted Uchiha eyes gets Rinnegan
Example: Nagato
- Maybe Tobi gave Nagato a Sharingan because he’s got the sage’s body, and therefore would get the Rinnegan(right away). Right now, Tobi got the rinnegan back, and uses it himself.
An Uchiha with implanted Senju DNA gets Rinnegan.
Example: Tobi and Madara
- After Madara fought with Hashirama, he got two things.
1. Hashirama’s DNA
2. The Rinnegan
He only fought to obtain the DNA. After he had the DNA, he also had the Rinnegan.