You're right in some respects. Fergie had less tolerance for under-performance, and was far more ruthless when a player didn't cut it.
But he's bought his fair share of pony in his time. Made his fair share of poor decisions.
Wenger is guilty of not being ruthless enough. And a top flight club should be more ruthless....but I still admire his trait of wanting to give talent a chance. Diaby is a case in point. If the lad could stay injury free, he'd be a monster, and the player we're all craving.