Frontierland in its original theme/core idea has been irrelevant for decades. You have to remember it was built in a time where cowboys were all the rage, there was the "cowboys vs indians", gun slinging was seen as "cool". This is no longer the case. Cowboys aren't popular, they aren't idealized and are now seen in a different light, and guns are not very cool to most younger audiences and fail to connect in a family friendly park.
It's simply times changing. People were upset about Tomorrowland not evolving with the new vision of the future. This is evolving to being about the American Frontier and removes the references and controversial nature. I mean a play area inspired by a book with slurs and racial stereotypes that no kid reads except as mandatory school reading... the writing was on the walls.