It's very difficult to stay a benevolent superpower forever. Very few countries have done it for long. Most superpowers end up being ruled by corrupt totalitarian regimes if you look at historical examples.
Not sure what made USA change for the worse or if it has just been a slow decline.
I don't disagree that Trump created a much worse environment for attracting talent from the visa process to general perception of the US but I think quite a few issues that make people hesitate about trying to move to the US came before him. I'm thinking about the difficulty of navigating most areas without owning a car, issues with healthcare, gun control, etc.
Actually it is not a democracy, nor was it ever designed to be one. Many people did allow themselves to be used though. It is pretty amazing how well propaganda works. I try to do periodic self checks, and I am still pretty sure that I have some mistaken beliefs that wormed their way into my head by stealth and/or repetition.
Not sure what made USA change for the worse or if it has just been a slow decline.