There’s a lot to unpack. Grade levels haven’t strictly decreased: 2008 saw a level of debate not seen in 20 years. The decline isn’t recent: there is a drastic difference between pre- and post- Reagan debates. Only 8 (of 33) debates saw a republican speak at a higher grade level than their opponent. The highest score went to a peanut farmer and Naval Academy graduate, the lowest to a billionaire and Wharton School graduate.
Perhaps it’s been a while since you were in the grade levels reached by these transcripts and aren’t sure what Donald Trump’s 3.69 or Joe Biden’s 4.51 should remind you of. Think “Charlotte’s Web”(3.44) and “The Outsiders” (4.25). When you see Jimmy Carter’s 12.4, think “A Brief History Of Time” (11.88). When you see that half of all debate performances are below 7.8, think “To Kill A Mockingbird” (8.01).