If a show is written well, there should be no need to stay "modern." And the idea that kids today have different requirements is a myth in my opinion.
Off topic, but that stuff is concocted by a bunch of hack child psychologists that just want to write random books so they can make names for themselves. Not every one is a hack, mind you... just the most vocal.
And IMO, SS has always been modern to some extent. I mean, in the 70's they did Disco, in the 80's they had Weird Al style song parodies... it's a very hip and up to date show. That's why I never feel ashamed to like it (see, we can weave things back on to the topic). It's just certain segments and ideas that keep it more for the younger audiences.