I never thought any generation was “stupider” because of the slang spoken. Hell, that’s like learning a second language. And I do admire you notion of what constitutes “Grampa and Gramma.” Teens speaking slang in the “1920s” would be, what, between 101 and 116? Makes me feel like the child that I am.
However, as for what I do regard as “stupider,” I read a post by a present day SJW who advocated strongly that President Trump should be impeached so that Hillary Clinton could then rightly assume her post as President of the United States. Now that, I regard as “stupider.”