Why do people avoid the fact that Sasuke's fireball was greater than Itachi's?
Same attack, differnt power.
Fact Itachi got overpowered.
Now regarding Itachi's Magenkyo.
We know that the Amaterasu comes with great abilities, Itachi himself even said that he gained all the abilties (Tsukuyomi, Amaterasu and Susanoo) the same day,
no training required.
As for the bigots, that refuse to believe that Sasuke would've won if Itachi didn't use the Magenkyo.
And are refering to this.
This doesn't prove anything. Especially, since Zetsu, a few panels later states that he can't put his finger on it.
Futhermore, it was Itachi who brought Sasuke to the tempel. It would've been a disgrace if he decided to show up weakened or unprepared.
And Sasuke had the chance, but never took it, to gain the Magenkyo in the valley of the end.
Zetsu, as seen above compared the Magenkyo to a tool of war, as a weapon.
Now lets say that I decide to bring a gun to a fight. Just because I can. Does that make me a better fighter?
As mentioned above, the three abilities that put Itachi on top came from the Magenkyo, and one of them had no effect.
So if Sasuke had decided to take it back then, he would have won this battle rather quickly.
And Madara wasn't suprized by the result of the battle either.
Sasuke is more powerful than Itachi, while Itachi possess more powerful jutsus, ebcause of the Magenkyo.