Honestly., I struggle to recall any example of hegemon restarting the engine after the onset of sliding down the slope. The sole exception in recent times is, naturally, China, who is now ascending to hegemony through interesting reenactment of the Warring States era in the planetary scale (e.g., they constructed and completed their own orbital station in some 4 years), but it hardly can be deemed an expansionist civilization, despite the fact that China dominated the world economy for most of centuries preceding the Industrial Revolution.

So I think that the decline of the US and especially EU after ceding the Mandate of Heaven will be irreversible lest they transform into some new vessels of existence, e.g., North American Union, Gran Publicamericana and so on. But the possibility of the next Civil War across the pond is quite large.

Disclaimer: I hail from Moscow, harbour a significant interest in Chinese history and culture, and greatly despise all current actors of the proxy-WWIII, to wit, Putin, Zelensky, Biden, Blinken, Borrel, von der Leyen, Johnson, et al. ;)