it's kinda ironic...because if the likes of madara and his followers were to have kept control of the clan.they would probably have wiped eachother out or everyone would eventually be blind from sacrifice,izanagi,izanami or just overusage.
I don't think the Uchiha clan was that willing to go through sacrifices. Itachi implied that Izanagi was mainly used during important incidents, and only a few began to abuse it to the point of another needing to use Izanami. Even then, those were eventually made forbidden by the clan as a whole. And of course, MS and EMS were only relatively recent additions to the Uchiha clan. And if we go by what Itachi told us about him, then Madara really was a good leader before becoming overconfident in his own power to protect the clan.
well...madara getting EMS...i wouldn't call recent.
as far as sacrifices...i didn't mean willingly...such as the way madara/itachi/sasuke achieved their power MS/EMS especially the latter...
Depending on how long ago the Rikoudo Sennin existed, it is very likely that Madara attaining EMS less than 100 years ago in the manga would be "recent' clan history.
Keep in mind the Elder Son of the Sage was the first "Uchiha". He probably didn't call himself Uchiha, but it would be his offspring that would form the Uchiha clan. Given the fact the Sage was considered a legend, I'd say he'd at the very least have to be something like 300 to 500 years old, possibly going back even further.
"The line dividing good and evil cuts through the heart of every human being. And who is willing to destroy a piece of his own heart?” - Aleksandr Solzhenitsyn
Definitely was longer. Remember, Fukasaku was surprised that the Rinnegan was real and he had been around for 800 years. So at the very least, we're talking about 2000 years plus for the Rikudou Sennin to have become a legend. Then again, Hashirama's power was considered a legend and not much time had pass since he had lived...
I doubt it's a merging. If that was the case, I fail to see why it would be referred to as a transplant over and over again-it's not like it would be a major shocking reveal to say "The eyes merge together, they're not replaced".
I'd find it more likely that either A ) The new wielder's chakra implants upon his brother's eye when it's implanted, thus creating the mixed design, or B ) There is no explanation; Kishimoto thought it would look cool, so he did it.