I think "Seinfeld" will age well. Unlike most of comedies today which deal with topical humor, most of Seinfeld's humor deal with universals: dating, relationships, food, slapstick.
Sitcoms that age well speak to people that have seen them, as well as younger viewers viewing them in re-run form for the first time. Originality is also a key. If a sitcom takes a run at a new topic, it will become beloved, like Lucy and the assembly line, and Kramer and his cooking or wacky inventions.
But "Roseanne" doing send-ups of "Gilligan's Isand" and "I Love Lucy" come across incredibly stale. Topical humor depends on previous knowledge for laughs, and that's not going to be the case decades down the road. Seinfeld had it occasionally, such as Kramer's Mary Hart lurch. (Now how many people are going to know about that syndrome when Mary Hart is too old and wrinkly to do ET anymore?) Thus, originality and universality are key to becoming a beloved sitcom.
Look at Cheers, it hasn't aged well. Most of it looks so 80s. It's almost kitchy in it's Reagan-esque jokes. And how about "Family Ties, boy, talk about your supply-side humor. Far too topical. "All in the Family" has too many Hippie-Nixon jokes to be relevant today.
Sitcoms still hilarious are I Love Lucy, The Honeymooners. Aside from a few things like real movies, answering machines jokes, I think people will still be laughing at Seinfeld 20-30-40 years from now.