Excellent theory and I agree
Also I believe that Elder Son had something smilliar to EMS, maybe more powerfull or even had First EMS.
and to some of you arguing that he would be forced to take his brother eyes ?
No, it wasn't necessary especially If Sage of six paths divided his/Juubi power equally with Creation of All things to create his sons aka First Uchiha and First Senjuu. Even If it was biological birth then still it could be explained somehow. It is called devolution form Pure and The greatest source, in other words Sage of six paths's Rinnengan got devolved into EMS biologicaly as reaction for lack of Senjuu DNA which was used to create Younger Son and it would make sense also.
Then Blood diluted over times decresing occular power of eyes devolving it to Mangenkyou Sharingan..., The reason for devolution could be lack of hatred or decreasing amount of hatred which was needed to awake stronger occular power in your eyes. Finally even MS got devolved to Sharingan... and Then somehow Byakugan if it is even included into that story.
However to evolve your eye you need to fulfill special conditions as it is always much harder to get something then lost something what you had already
It could explain that they are two ways to get Sharingan, MS, EMS and Rinnnegan and maybe even Byakugan if it fit into story:
1) Be born already with these eyes
2) Reach and fulfill special conditions to get them:
- Sharingan - have hatred in yourself and be skilled enough, maybe strong genes as well
- MS - expierience death of close to you friends... either killing him/her by yourself or watching it... suffering cause increase of hatred which lead to increase occular power in eyes
- EMS - transplant your brother MS into your own eyes getting eternal light
- Rinnengan - having EMS and implanting Senjuu DNA in yourself also expierience enough suffering and knowledge about The World
This is how I see it works....
Elder Son was born with EMS in this case, while Sasuke, Madara, Izuna and Itachi, Shisui and Obito had to evolve their eyes reaching special conditions.
Remeber devolution comes really easy, but Evolution of something take much more time and much more efforts, as it is hard to gain something while it is easy to loose something.
I hope this clear up a bit
EDIT: actually it even make a lot of sense:
Sage - born with the Rinnengan (Juubi's eye while being it's Jinchurki )
Elder Son - born/created with the EMS
Younger Son - born/created with the Senjuu DNA(Wood release most likely)
First Uchiha(maybe Son of Elder Son) - born/created with the MS
Uchiha from old times - born with the Sharingan
Modern Uchiha - Normal eyes with potential to awake Sharingan and evolve it.
It looks easy to understand, however it is hard for me explain why devolution even was happening. I mean Rinnengan -> EMS + Senjuu DNA(Wood release) is understandable, but why EMS could devolve to MS... maybe due to lack of hatred or forgivness... but then why MS degraded to 3 tomoe Sharingan... lack of hatred again... or maybe too much love, lack of negative feelings ... times of peace and lack of war... probably and finall Sharingan-> Normal eye with potential to awake Sharingan. Of course the blood diluting could be blamed but could only this ?
What do you think OP... what would cause devolution eye form EMS-> MS, MS-> 3 tomoe Shairngan, and finally 3T Sharingan-> normal eyes with potential of awakening 3T Sharingan ? please tell me I am interested