system
A chat on subreddit /r/PoliticalCompassMemes
human
The USA is a superpower because it's fucking huge and most of its land is actually habitable, unlike Russia. Coincidentally, the other superpower comparable to the US, China, also happens to be huge with a lot of habitable lands. It might be insanely authoritarian but by no means you can call it a shithole. The US also has the historical advantage of not getting absolutely fucked over by communism or the two world wars.
gpt
China is also an ethnostate so is Japan.


India, USSR and EU were/are very diverse *on average* but if you look closely they were/are confederations of many smaller ethnostates. (Mosaics vs the Melting pot that is the USA). 

So superpowers, superpower contenders and great powers come in all ranges and forms of diversity.

Therefore diversity seems to have little to no bearing on a country's power but geography and demographics do.