This is actually a pretty good point, indirectly. The rise of widespread internet culture and meme might have something to do with this feeling. Kids are brought up with international internet lingo and jokes that are ubiquitous and timeless. You don't need to know that joke X comes from old movie Y, you just know joke X and you know it's funny. It permeates your culture regardless of your age, and that creates connection between past and present generations.
As the quote goes: "Hamlet is just one famous saying after another"
I can watch 20 other sitcoms from 10 and 15 years before Seinfield (from MASH to Mork and Mindy to Taxi), that I wasn't even adult when they first run, and they are still funny, even when their styles and jokes have been copied....