Funny how a when something is successful there are a number of imitators. Right after Seinfeld was finally a big hit, the short lived show "The Single Guy" came around about three (or was it two?) guys and a girl in an ensemble cast. Also the Drew Carey Show about three guys and their "one of the guys" female friend.
I'm sure there are / have been others.
Sad to Seinfeld go, but its best to go out on top, and not be known as a shadow of what one used to be. Take for example boxers or athletes who just don't know when to quit- its almost an embarressment to see them perform.
Even though the show is about "nothing", there will be a shortage of "nothing" ideas someday. I'll miss George's hard-luck, Kramer's wild ideas, Elaine's inability to tell a lie, and most of all, all of Jerry's girlfriends (Teri Hatcher, Jaime Gertz, Tawny Kitaen, Lori Laughlin- to name a few).