You know, not all Leftists think like that.
The definition of Leftist is anyone who holds left-wing political views. This refers to a wide variety of people, from moderates to radicals, a whole half of the political spectrum. Two entire quadrants of the political compass, in fact. Are there some Leftists who do think like that? Of course. Do all of them think like that? No, of course not.
Just as the existence of alt-rightists doesn't make the entirety of the right half of the political spectrum fascist, the presence of alt-left doesn't make the entirety of Leftists communists for example.
"The flaws and failures of the people trying to live up to the promise does not change the promise itself. In fact, it is those very failures to live up to the promise to which people can point and say "we're supposed to be better than this!" It is what inspires people to fight for the promise, rather than settling for less."
Exactly! This is what America is all about, and it's a huge reason why I'm proud to be American.