I am talking about the 20th century where all countries lost their colonial empires at around the same time late 40s-70s, you are referring to a much earlier period

Britain, Spain and France still have parts of their empires

Spain, Portugal, France and the Netherlands lost their superpower status before the start of the 20th century

if you're in the year 2300, maybe the state Spain and Portugal ended up several hundred years after the loss of their superpower status is of interest

but today, the question we are interested is in "what happens at the point the US loses dominance over the world?"

will the US will handover the reigns to China in the way Britain did to the US?