The problem with narrativized framings like "Pax Americana" is that they only work if you focus on internal peace. The "American" century began with World War 2 (arguably) and was defined by continuous proxy wars and assassinations. The US also didn't stand unchallenged at least until the decline of the Soviet Union (remember: the commies even won the "Space Race" before the goalposts moved to putting a man on the moon) but arguably that was also a crucial step in the rise of China as a direct challenger.

In the case of Pax Americana the framing is also dubious as it wasn't American dominance that kept the peace in Europe (on this side of the iron curtain) but arguably more the shared market and the necessity of cooperation to recover from the wounds of two world wars while facing the threat of annihilation in the conflict between the US and the Soviet Union.

Even in Europe this period was heavily defined by oppressive policing in both East and West Germany (culminating in the fall of the Berlin Wall in the East and the student protests and RAF terrorist attacks in the West), civil war in Northern Ireland (with terrorist attacks reaching deep into England at times), separatist movements in the Basque region, the excruciatingly slow death of fascism in Spain and Portugal, the violent suppression of striking miners in the UK, and the birth pains of neoliberalism and austerity.

The "pax" in these titles always only applies in a very narrow sense to the affluent in its imperial core, i.e. the American upper middle class of the 1950s or the British bourgeoisie of the colonial era. Even the Pax Romana is not a coherent description of life in the Roman Empire for the time frame it is often applied to and was defined by expansion (i.e. military conquest) not an absence of war.

If anything, the "prosperity" these terms often imply always only existed because of a hierarchical system of exploitation and the "peace" refers to the absence of serious challengers to disrupt this exploitation. The prosperity in Britain during the Pax Britannica specifically only existed due to the violent oppression of British colonies and the absence of powerful challengers to claim those colonies instead. Following the war economies of WW2, the 20th century saw a massive redistribution of wealth and public infrastructure to the financial elites, especially under Reagan in the US and Thatcher in the UK, while colonialism largely evolved from the crude brutal oppression of e.g. the British Raj to loans and privatization, aka "soft power" (promoting the production of worthless cash crops for international trade at low margins instead of vital food crops, making the economy dependent on imports to keep the local population fed, or exporting raw resources rather than building up local infrastructure to refine those resources into goods that can be sold at a higher price and thus having to import the finished goods at exorbitant prices).

So, yes, for you or I living in the imperial core - whether literally in the US or by extension in Europe - the "decline" and the rise of challengers is worrisome and can only be negative. But ultimately, especially to those living outside that core, the challengers are no worse or better than the status quo.