Therein lies the issue? "Fredo had no real role in the Family" not even figurehead, face-saving role

Fredo put up with whatever was dished out by his father Vito older brother Sonny and being straightened out / slapped around in public by "we're good friends, right Moe" but couldn't handle his kid brother, Michael being the Don because it should have been Fredo

If my memory serves me right, Vito was cruelly and contemptuously dismissive of Fredo in the book as well that Fredo was never the man for the Family business – Fair enough Fredo was never a contender for the Donship and was always going to be stepped over
Sure thing no one is disputing it! However....

The Great King, Vito never foreseen any issues with anything Vito was putting in place
It never even occurred to Vito, Vito never even considered Fredo [and Connie] may not go along with what Vito had tasked with Michael, let alone

Vito "didn't view Fredo as a potential threat/weakness" No Garden chat with Fredo "but he should have especially after the Vegas outburst" This failing of Vito, left the door wide open, with a welcome mat! for Roth

Vito was slippin' indeed same as underestimating Sollozzo