Very much disagree! Pilots who struggle through their training are often totally ignorant of how useless they are as aviators - I've seen this over many years and outfits Give me an ace F/O over a useless F/O ANY day of the week
I take your point but in my experience an ace who finds it all too easy is all too often taken by surprise when it goes pear shaped. Someone who has had to work for it has seen it before and takes nothing for granted. It is a matter of degree though. Essentially I'm referring to someone who isn't useless but has to work harder than the aces of this world. As a result they take nothing for granted. Frankly that applies to most of us. Not everyone is a potential Red Arrows pilot.
People vary in their capabilities, hardly a profound statement. But such is life. Good pilots can be born or made.
But you are right, a genuinely useless pilot can get through, quite often they are oblivious to their uselessness even though it's glaringly obvious to all around them.
My point which I really didn't make clear was that being a good pilot isn't simply about sailing through the training and having it fall into place. It's also about attitude and taking the job seriously.