All those shows are contemporary to the 90's. Seinfeld just has generic situational humor + great characters, and even Seinfeld will probably fade as its nostalgic watchers age and go away. It will just take 30-40 more years :-)
I don’t think 20 year olds will have an easy time picking up the show as they can’t relate to those problems.
I think Seinfeld is very much riding the nostalgia train now.
Mad men don’t assume any of its audience knew what the 60’s was like, so it has a chance of being more timeless.
And to the original topic, a good LotR series would be closer to "timeless".