There are a handful of academics who say America is irredeemable. There are a handful of fanatics on the other side who say America is a savior nation (as Limbaugh does) -- a position I, as a church elder, regard as open heresy.
The vast majority of academics are engaged in the search for truth about their topic, whatever that topic is and wherever the search for truth takes them. As teachers, we necessarily have to address the simplistic ideas that students bring with them. Some things they believe are outright myths. Teaching them that American history contains bad things as well as good ones is an essential aspect of good teaching and good truth-seeking. Some students (and quite a few non-students, like Limbaugh) resist having their idols questioned. This leads some academics to use very forceful language to insist that the bad things really did happen. I still get students who were taught that the Civil War had nothing to do with slavery. There are still students who are taught that racism ended in the '60s.
I know as a sociologist that is it very hard for most people to grasp the idea of social structures. We tend to reduce all social phenomena to individuals intending their individual action. This makes teaching challenging. But pointing out that bad things did happen, are still happening, and have left a structural residue which continues to have effects beyond what any individual intends, is not the same as teaching that America is irredeemable.
Limbaugh, who makes his money from sensationalized fear mongering, probably does know better (he has admitted as much in his several divorce proceedings), but it would interfere with his business model to admit it to his "dittoheads."
[This was written in response to a friend asking about a specific Rush Limbaugh show from October 13, 2020, but repeats a theme he has expressed often.]