I Think EMS Was Indras Natural Eyes.

[Guitar Hero]

His MS probably didnt deteriorate.

 

[SenseiSama]

Yes, he had Sage chakra which would mean he had the same MS feats as Rinnegan Sasuke. Furthermore, Kishi likes being consistent with division of power, Hamura + Hago = Kaguya, Ashura += Indra = Hago, Senju + Uzumaki = Ashura, so it's only logical that MS + MS = Indra's eye power.

Uchiha have diluted Sharingan power so they don't have Indra's chakra potency. It wouldn't be out of reach to say a transmigrant and their sibling are born with equal halves of his power but only the transmigrant can unlock EMS

Itachi had Amaterasu like Indra's transmigrant and Tsukuyomi which happens to come from RinneSharingan which is the origin of Indra's power, and after the transplant Sasuke stated he could feel Itachi's power growing in him

 

[WalksInShadows]

this is news to you? :|

 

[oxtopus]

Ms,it could be possible that he had every sharingan ability ever shown + more since he was like the progenitor of the ms