The trilogy came up with lots of surprises. Before the death of Vito nobody would have thought that Tessio would turn traitor. He was an oldtimer just like Clemenza.
And from what we saw in GF, it would have been "implausible" that Michael would have his own brother killed. "Audiences would never have bought that".
The whole film GF2 is to show how Michael isolates himself more and more from the people who meant something to him: His brother, his wife, and the last friend of his father.