It wasn't ever that, really, clo.
It's a good idea not to overindulge the national mythologies which are part of the root of the problem. First, the shining city on a hill was an exclusive puritan fantasy and the US has only occasionally been an example to the world. To begin with, it was a colony which decided it wanted independence, which is fair enough, of course. It also ran a slave economy, and had its eyes on land owned by indigenous tribespeople. Later the US was a great power, which meant that its influence was massive. But the beacon thing is a self-made image, which is concealed self-aggrandisement. Trump embodies a bloated, orange version of that narcissistic way of thinking.
Second, it's those fantasies which left the constitution so undefined that a man like Trump can do what he does. There are assumptions about the greatness and morality of American presidents - even Nixon resigned to protect the dignity of the office - which left blind spots. These things need to be remedied when Trump goes. He provides the exceptions from which to learn.
Frankly, the chief executive is too powerful. The US lacks an effective trust architecture in which to restrain him.
In its heyday, America became a place many people went to go to, in order to have a new life. And many immigrants made America better by coming. They were the beacons which brought light to America.