I reckon the other factor to consider is complacency which resulted in these people slippin'

Vito was sitting pretty having made loads of money, with influence and power as Top Don
I agree he didn't need the hassle of "a dirty business," money

The slippin' Vito failed to see, Tattaglia and Sollozzo still climbing up the pecking order ladder, hungry for money and power, wouldn't just walk away from millions, $10M estimated, in just the first year alone

He suspected there was more to what Sollozzo was telling him, yet never occurred to him the possibility of a hit
Ten years ago could they have gotten to him?