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The vast majority of people I see declaring we’re #1 are taking credit for it but not working to be the best in the world. It’s the people who are saying we shouldn’t help other people outside the US (or even inside). It’s not a declaration of intent for most people. It’s a badge of pride that they don’t deserve and don’t intend to maintain.
If it actually were used like you imply, I don’t doubt we would be the best. It would require us working to improve the world around us, not just benefit ourselves. Usually it’s said out of nationalism though, not optimism.