I really hate nominal GDP per capita because it leads to these distortions (high oil prices for Iran). This isn’t true. Iran had an extremely high illiteracy rate during that time. It was never going to end up like South Korea despite what certain dissidents claim.
The US is not in decline but the US alliance system is in rapid decline. Two very different things. US allies (except Israel and Turkey) are much weaker now both economically and militarily.