"American Hustle" exposes the manipulator in all of us. That's why it is resonating with audiences. It frees us from the Boy Scoutish/Girl Scoutish notion that we are expected to play the game straight. Now we can fully embrace and come to love our inner Machiavelli
Some of the characters, like Irving and Lady Edith, go heavy on the manipulation. The rest, like Irving's wife, just do it when they see it as totally necessary. And some are better at the game than others. But everyone is in full control of their inner Machiavelli. Even Irving's adopted son knows the wrong things to say at the right time.
Yet, true relationships can endure. Irving and Lady Edith, reinvented from Sidney, do pair off, just as in a Shakespearean comedy. Irving truly regrets having to screw over his new pal Carmine and does intercede so that Carmine will only have to serve 18 months in the slammer.
Also, often manipulation is the only efficient way to get things accomplished. Irving's first wife wouldn't have gotten out of the marriage without some temporary serial conning, including of herself. She sees herself as helping Irving grow up, not falling for another guy.
But, probably, most importantly, hatching and executing a Machiavellian plan is a lot of fun. Irving and Lady Edith are not kids. He even has a belly and a bad comb-over. But they live their lives with the passion, expectation and excitement of youth. Perhaps those over-50 need a seminar in the Fundamentals of Machiavellianism to ensure they can retain a zest for life.