declinist
noun
de·clin·ist di-ˈklī-nist 
dē-
: one who theorizes that a nation or society is in or is headed for a state of economic, political, or social decline
Example Sentences
Recent Examples on the Web And contrary to increasingly popular declinist narratives, the U.S. will be at the forefront of mankind’s second sustained push into the final frontier. 
Andrew Stuttaford, National Review, 21 Nov. 2020 While the United States has defied many declinist predictions in the postwar era, there is good reason for recent anxiety. Ali Wyne, The New Republic, 21 June 2018
Andrew Stuttaford, National Review, 21 Nov. 2020 While the United States has defied many declinist predictions in the postwar era, there is good reason for recent anxiety. Ali Wyne, The New Republic, 21 June 2018 Word History
First Known Use
1988, in the meaning defined above