We have walked away from what made us great. Every great power I can think of in history that walked away from what made them a great power has completely collapsed. I cannot think of any that have peacefully transitioned from one type of successful society to another. Perhaps Great Britain, but that was also driven by the two worst wars in history and still caused her to fall from the status of a great power.
Athens was a great power due to their navy and empire. During the war with Sparta, they built up a land army and rested on their laurels with their navy. Sparta built a navy, took the empire from Athens and Athens has been a marginal power since. Ironically, Sparta's change from a small elite land army to a larger naval power led to her collapse.
Similarly, Rome became great largely due to their style of government and incentives for conquered peoples to profit from joining Rome. After Cesar, the government slowly grew to be completely out of control (sound familiar?) and Rome collapsed.
The United States became great because of a combination of a limited but effective government, the work ethic of the citizens, the ideal that anyone could rise as high as their talent and effort merited, an abundance in natural resources, and, most importantly, a strong sense of Judaeo-Christian values even if the founders did not always profess that faith. Today our government has grown so far beyond what it started that the founding fathers would almost certainly rise up in rebellion if they were somehow transported in time to be now. The work ethic of our citizens appears to be at a depressingly low level and I personally know healthy, capable individuals that would rather sit at home on unemployment than take a job they were offered. The new 'equity' movement gives the appearance that skin color is more important than merit. Even if you do well, you have combined tax rates pushing 50% in some states that drastically reduce the appeal to work harder to earn more. Finally, we are willingly stopping from using our own natural resources in favor importing those exact same natural resources from other countries. Most importantly, the Judaeo-Christian values that this country was built upon are being actively attacked and portrayed as hateful. Those values are/were the bedrock of the American family which is a microcosm of America in general. Without that traditional American family unit, you cannot expect children to grow up and endorse those values in any significant percentage.
As others have mentioned, you cannot rip the foundation out from under a house and expect the house to stand. We are living in a house where members of the household are taking jackhammers to the foundation. If this goes on for too long, the house will collapse and take many of us with it.