Soviet Union in terms of a similar, modern-era faster pace of decline and types of milestones.
Western Roman Empire in terms of the final outcome and fundamentals.
As I don't think there will be a reunification again as in Chinese dynasties, or even some form of a leaner, weaker successor, as a Russian Federation. There will be permanent fragmentation as after the WRE too.
UK also survived by latching onto the US as a first tier vassal due to their affinity, and I don't think the US will do that with China to survive and keep a certain level of prosperity.