This is a universe where the moon is hollow and was created a thousand years ago to hold the corpse of a giant monster that was born when God's Mom fused with a tree becuase she was pissy that her kids were sharing their toys.

Scinece has it's place, but there's a very clear spiritual/metaphysical aspect to it.

he Rinnegan ins't really supposed to be in mortal hands. Kaguya keeping the fruit for herself and having kids with the locals isn't normal(apparently she originally decided that her clan was evil and decided to keep the God Tree's power for herself to protect the Earth from them... But then she went insane from a combination of power-drunkenness and paranoid) so if there's a "true" rinnegan it would be the form that the Outsusuki's gain from drinking the Cinnabar Elixir, since that's basically what they want the god-fruit for.

In this case, it's very much a... Basically, spiritual alchemy is the best way I can explain it. I mean, after Orochimaru's soul was permenealty sealed away in a realm of Drunken Dreams, Sasuke e was able to make an entirely new Orochimaru with all of the memories of the original from a chunk of Kabuto's flesh and a fragment of Orchimaru's soul that got split off like, 20 years ago(and thus, shouldn't have the original's memories.) This copy and the original retreated as being the same person fro all intents and purposes.

The same is true of Kaguya: Combining the Rinnegan, the Ten-Tails, and the chakra of a crap-ton of people in one human recreates Kaguya's chakra signature and allows her to return to life.

(This is why I joke that someone needs to put a stop to Sarada's crush on Bort. Between the two of them they should logically have enough traces of Sage power and tailed beast chakra that their hypothetical offspring would have most of the ingredients needed to form Kaguya once more.)

The Sage's rinnegan is listed as a Kekkei Mora, which means there's something more to it than just DNA. mixing Uchiha and Senju recreates the DNA, but it's the chakra of the Sage(or the combination of the chakra of his sons) that has the something extra needed in spiritual matters.

If it helps, if we take Kabuto at his word the combination of Jugo's sage mode, Snake Style Sage Mode, and the mixture of Kabuto', Orochimaru's, Kimmimaru's, Kidomaru's, Tayuya's, Sakon's and Ukon's, Jirobo's, Karin's, Suigetsu's, and Jugo's chakra natures is pretty close to The Sage's chakra, so it is theoretically possible to recreate that power without those two specific chakras. (He claims to have come closer to the Sage than anyone else, but he doesn't really know about Madara's self modifications.)

Apprently, being the Ten-Tails Jinchuriki(Specifically, having the chakra of the Ten Tails isn't enough) can also grant that power an you keep it if you lose the Ten-Tails.

If I had to give a reason, I'd just say that the divinity that Ashura an Indra inherited from their father was further subdivided over the intervening generations, the same way that they each only inherited half of their father's power. In which case, the "full power" could be replicated by combining the DNA and chakra of multible Uchiha's and Senju instead of just one of each.

As for destiny? That's the thing, there isn't one.

The term "Transmigration" is used to refer to it, but it is not literal Reincarnation. Naruto and Sasuke were not born with Ashura and Indra's chakra. They are not those souls reborn(otherwise, Hashirama and Madara couldn't have been brought back with Impure World Reincarnation because their souls wouldn't have been in the Pure Land.)

They inherited the chakra of Ashura and Indra by being the descendant in the current generation who was most similar to them: Naruto is talented, yes, but like Ashura he didn't inherit the impossible talentedness or special powers of his awesome parents. For years, the only thing special about him was the Nine-Tails, which was a mixed bag at best, and he was only able to achieve his full potential via a combination of making lots of friends and periodically working his ass off. He favors teamwork and cooperation.

Like Indra before him, Sasuke is an egocentric genius prodigy who crazed power above all else and is prone to using a power and brute force to solve the immediate problem. He is manipulated into betraying his friends by schemers who play on his psychological issues.

Madara was much the same as Sasuke, but Hashirama instead has Ashura's blind optimism and willingness to see the good in everyone.

Becuase of this similarity, eventually they inherit the power of their respective ancestors.

Kishimoto included it, basically, to give a reason for Edo Madara to sacrifice being an invincible zombie that instantly recovers from all damage and never runs out of chakra or stamina. If Kabuto's false Rinnegan was as powerful as the "real" one then Madara could have just plopped the Ten-Tails instead his zombie body and cast the Infinite Tsukiyomi before Sasuke even got there and then he would have won.