As I heard Larry David say in an audio commentary for one episode. The comedy in Seinfeld is more or less timeless. Sure, the fashion, brands, trends and technology are different now, but the situations and interaction between people are still easy to relate to.
The thing is that's what people do. The next generation will always come along and shit on elements of the past. Because they are out of the context of when that thing was created and also because they are trying to make their own stamp on the world. In 30 years Seinfeld will be treated like the way we treat I love Lucy now. As some interesting relic from a past age. That's just what time does.
Seinfeld was great and is important because there wasn't really a sitcom like that before. But now its so common, its not a big deal.
Its like the Beatles. It's why some people love them now and still marvel at them and other people think they are crap or "over-rated". It happens to everything.
I've been watching Cheers and it still "holds up"
Also as far as "holding up", you have to realize what a bland, commercialized, meritless, lowest common-denominator POS medium TV has been until the last decade. Anything of quality was something that somehow managed to be still in that forumulaic box without being formulaic crap.
To those that appreciate the strides TV has made in the past decade or so, there aren't many things that can survive, at least not enjoying them unqualified or "for their time".
I think it's a bit more merit to finding older shows hard to watch as TV is an immature medium that is finally maturing. It's not just one generation scoffing at the previous generation's fashion.