I'd say the weakening of prudishness is recent phenomena and it's too soon for it to have an impact on society's success. We'd need to wait a couple hundred years to see where it leads.
US is far less prudish than median. If you compare it to other developed countries, sure, it's on the more prudish end. But if you compare it to most everywhere else, it's a whole different story.
For example, Sayyid Qutb - the grandfather of modern Salafism - became radicalized after visiting US in 1940s, and observing its culture. Here's how he describes it in his writings:
"The American girl is well acquainted with her body's seductive capacity. She knows it lies in the face, and in expressive eyes, and thirsty lips. She knows seductiveness lies in the round breasts, the full buttocks, and in the shapely thighs, sleek legs -- and she shows all this and does not hide it."
"They danced to the tunes of the gramophone, and the dance floor was replete with tapping feet, enticing legs, arms wrapped around waists, lips pressed to lips, and chests pressed to chests. The atmosphere was full of desire."