The rinnigan only evolves from the Sharingan if the user somehow gets or does thing with senju cells or what ever right? Why it evolved after his fight with senju. Is it not somewhat stated or somewhat suggested that Senju and uzumaki are closely related in some way? In the very least Uzumaki have special Chakra! It was stated even back with narutos mother. Maybe that has something to do with how or why nagato can use the rinnigan so much. Plus to me it seems like the rinnigan more so grants the user more control and chakra ability instead of taking it. Unless the user uses the more advanced abilities of the rinnigan. Nagato could use all Ninjutsu elemental types right? Was that ever directly related to the rinnigan. It was highly suggest at one time. Could that be more uzumaki related? I don't think it is but they have not fully explained that aspect I don't think. Nor the whole chakra controlling black rod things that naturally sprout... or the demon mazu type susanoo thing that Nagato summoned during the flash back with Danzo or even the hole paths and multiple body thing.
We don't really know anything about the rinnigan do we? Except that it probably evolves from the Sharingan? Even that fact is confusing. Well and it shares vision and grants a lot of special abilities a lot of them seem to be about restraining something. The rods control/suppress things, it has the ability to pull and repel, then it has the whole almost sealing ability that apparently created the moon etc.
The rinnigan almost felt like it was created to defeat a massively powerful beast! Were as the sharignan seems more natural and bent on pain, misery and destruction. Since the point were Madara evolved his sharingan into rinnigan I have been lost to trying to understand them.