Allowing Madara to keep his Rinnegan after being resurrected is dumb. Why? Because this logic has made it possible to farm Rinnegan for free. This is how you do it:
Prerequesites:
- Someone who's unlocked Rinnegan
- Someone willing to die a ton of times with you
Procedure:
1. Madara transplants his eyes on Obito.
2. Madara commits suicide / is slain by Obito.
3. Obito, who now has Rinnegan, uses Edo Tensei on Madara, reversing him to the point when he had Rinnegan.
4. Obito now sacrifices himself by using Rinne Tensei on Madara and dies. Madara is now alive and has a second pair of real Rinnegan.
5. Madara extracts dead Obito's Rinnegan and sets them aside (these are the first farmed Rinnegan).
6. Madara uses Edo Tensei on Obito, reversing him to the point when he had Rinnegan.
7. Madara now sacrifices himself by using Rinne Tensei on Obito and dies. Obito is now alive and has a third pair of real Rinnegan.
8. Obito extracts dead Madara's Rinnegan and sets them aside (these are the second farmed Rinnegan).
9. Repeat.
What to do with this?
Madara would probably keep them all for himself since he's a selfish *******, but what if Sasuke unlocked the Rinnegan and did this with Naruto? Free Rinnegan for all of the Shinobi Alliance. 40 000x Chibaku Tensei. GG Narutoverse.