I was going to go into it but Zero pretty much nailed all the points on Shisui's eyes and on what Itachi lacked.
The Reason Sasuke & Itachi are so strong are because their parents marriage was carefully selected as Fugaku Uchiha was the head of the clan meaning he had some of the strongest traits. Mikoto Uchiha was probably from another high level stock of Uchiha thus their children were passed on powerful genetics.
To me it seemed like the Uchiha's bloodline seemed to fade away, since not every clansmen had a sharingan anymore.
- After the Uchiha discovered Izanagi more and more members began to rely on this power to sort of correct their mistakes.
-Which led to a kind of careless arrogance within the Clan as they figured hey if I screw up I'll just use Izanagi and choose something else.
- Izanami was created to teach those Uchiha's who started playing around with Izanagi a lesson.
- Because Izanami creates a loop which one can not escape until one realizes ones true self the Uchiha who were used to cheating their mistakes with Izanagi were forced to accept their limitations and recognize that one cant rely on tricks to escape their destiny.
- Thus the Uchiha who managed to escape learned their lesson and the Clan labeled Izanagi a forbidden jutsu
Mikoto had two sons so I would guess most married females gave birth to several kids in order to maintain the clan. That and perhaps some of the lower Uchiha women had multiple male donors acting as some kind of surrogate mothers.
The really implausible part of Itachi's explanation regrding Izanagi and Izanami is this, How can it be possible for someone spam Izanagi and get all huffy puffy, when Izanagi makes you go blind. An Uchiha should only have two shots at Izanagi before going totally blind.
The implication here is that Uchihas have been robbing the eyes of their dead or their neighbors, perhaps they each even have personal stashes of eyes stored away just like Tobi does. If this is true, then theUchihas are and have been a Sicko bunch of weirdos for a looooooooooonng time.
I am not perfect and I defy you to prove otherwise
Growing Old Gracefully is an Oxymoron ... Mostly Moron !
Am I the only one who is waiting for the hillarious scene when Madara is released from Edo Summin to relief from the kages and the whole army, only for him to be summoned by Tobi(who now knows Edo) 5 seconds later to general what-the-fuk and we're-doomed faces from everyone
In regards to the Izanami conversation...
I believe Itachi is purposefully hiding something form Sasuke, witch is why the explanation is not complete.
Take a look at the graph showing repeated use of Izanagi ending with an Izanami loop, that graph shows three uses/attempts of Izanagi. Yet without transplanting their eye's an Uchiha should only be able to cast two. Moreover the way he word's it, It feel's perfectly logical that someone possessing such power would sooner or later fall under the illusion of invincibility (because he can always choose the better result of the two). Yet someone who can use only two Izanagi's in the entire lifetime, and for the cost of his eyesight, would probably never arrive at such a dangerous thought.
I think that the knowledge of how to transplant eye's in the first place, was probably developed for the purpose of restocking the eye's of those who lost them while performing Izanagi. And it only later became a problem on it's own (an entire generation of eye-stealing Uchiha) not to mention that it was used as a 'cure' for the side effect of MS.
As for Izanami, i can envision one of the higher-ups of the clan to be dying because of a mortal wound, with perhaps only hours left to live. The Uchiha then would gather, and essentially chose his fate. If his life was worth the price of a single eye of some lower Uchiha (or perhaps, a dead one, who no longer needs an eye) then why not? But what if they disagreed on the matter? What if most of them argued that he's old and should just "accept his fate"? Or maybe the higher-up stood in the way of some sinister plan. Either way, it's easy to imagine someone storming out shouting: "I'll use Izanami then, you fool's! And he will know how much you value his life!" Only to be stopped by someone "If you dare to use Izanagi, I will not hesitate in using Izanami!"
Izanami was in my opinion, a technique to enforce. It's ridiculous to use it to 'save' someone from feeling too powerful. It cost's an eye after all. But if it was developed solely as an anti-Izanagi super weapon, than it's quite powerful in it's own right. It was propaganda, nobody ever had to use it, it was enough it existed. I would certainly think twice before using Izanagi if someone had a technique to counter it, because that would mean i just lost an eye for nothing.
Of course Itachi is still hesitant to tell Sasuke how bad his beloved clan really was, because it was always Sasuke's dream to rebuild the clan. And that's a pretty beautiful plan, that may never be realized if he were to know the 'truth'.