Politics aside it does seem that presidents have been pitching their language lower and lower to appeal to a broader audience. I recall reading about how Obama simplified the vocabulary of his speeches from college level to 10th grade, to 4th grade to 2nd grade: https://smartpolitics.lib.umn.edu/2010/01/29/professor-obama... https://smartpolitics.lib.umn.edu/2011/01/27/keeping-it-simp...
On the flip side though George Washington's first inaugaral address goes to the other extreme where it's impenetrably verbose: https://www.theatlantic.com/politics/archive/2014/10/have-pr...
Anecdata at this point but I fear the OP might be right - we are simplifying our language over time.
