So Itachi decided not to kill him and save the world because......?
I'm sure Itachi would've been warped into Tobi's dimension long before then.
Which turns out to be crows, Itachi is still there... has cought Tobi in a Tsukoyomi and trappes him with izami, cancelling out his intervals of materializing . Sharingan lost. Tobi gone. Tough victory, heavy losses, not enough archieved.
Or choses to play along. Protecting Konoha is his own way. Being a living barrier between Tobi and Konoha's destruction(if wanted).
I think Itachi made decisions in his own very logical way. Being very analytic, and aware of the situation at hand, he was capable of seeing a patern, and how it would evolve. A true genius with the Sharingan. However, he never was the one to wish to remove all hate from the shinobi-world. That, was never within his patern of logic. All he did was to do what seems best to him, to a greater picture. He acknowledges this whilst his last goodbye with Sasuke. If only he confronted Sasuke with the truth, instead of treating him like his little brother.
Itachi would pretty much do well against any shinobi. Itachi's worst enemy is himself.