American exceptionalism is still alive in today's world, even as other countries surpass the U.S. in various metrics. Every day American citizens discover new things and raise the bar for human achievements. Therefore, exceptionalism isn't going to disappear anytime soon thanks to the citizens in the United States that continue to strive for more.
American exceptionalism is not a myth, it is just lost. The idea is still there and in a perfect world it could be acheived. Sadly with so many people greedy and power hungry the pursuit of acheiveing it has all been lost. Maybe one day the idea will come closer to a reality.