It's hard to say because the US was, by far, the most dominant economic power in the world in decades past, and now we're losing that role, or already have depending on how you measure things. But the nature of going from #1 to #1 (or #2 as it may be) means you have no real basis for comparison how things might have been.

But what I'd observe is that by many metrics the US hasn't really made major progress since our glory days. For instance real median wages from 1979 to present have increased by less than 13%. [1] It'd be disingenuous to compare that to China because they started from much less so obviously you expect dramatically higher growth, but I think it's reasonable to claim that we could have, and should have, done dramatically better than 13% growth in almost 50 years.

Another point is that the US was able to 'artificially' strengthen its economy owing to the dominance of the dollar and being the center of the tech boom. I say artificially because those things were obviously going to be liminal. We needed to leverage those advantages into something long-term. Instead it looks like we've been mostly treading water and largely wasted it all.

And now pair all of this with contemporary times with a sharply divided population, unsustainably low fertility rates, terrible education outcomes, and more. And no, I don't think it's reasonable to claim that the US is doing well, let alone to the point of being able to rest on our laurels as you're implying.

[1] - https://fred.stlouisfed.org/series/LES1252881600Q

While China's improvements are amazing, don't be fooled, overall China is still behind. They have more people and so totals can look good, but the common person has much improvement left before they beat the US. Ultimately, I think your argument comes down to, when you're on top, it is very hard to improve much more, while, when you're starting at the bottom, you can improve a lot very quickly and easily. Indeed, China's growth numbers have been slowing a bit recently, which supports my thesis. Only time will tell of course

Also there is plenty of reason to think that China isn't giving honest numbers, but what's the true numbers are is impossible to know

Behind by what metric? PPP their economy is already much larger, their educational outcomes are much better, they're doing wild stuff in bleeding edge domains like robotics, built/launched their own space station super rapidly, and so on. Pretty much the only domain they're clearly behind in is rocket technology owing to SpaceX, and back here stateside you have plenty of people that hope the company somehow fails because they don't agree with the political opinions of its founder.

And the numbers we're citing aren't just from China, but from third party organizations. In any case, the arguments don't come to debating numbers - the change is obvious and visible. It's not like they're claiming new record low unemployment while seemingly large numbers of people somehow people struggle to find jobs.