According to Seinfield's biography, Elaine was an amalgamation of several different women he had known or dated previously. Also, the original pilot was guys only and the network felt it was too "male-centric" and wanted at least one main female role added to the group so women might actually watch the show.
Elaine was one of my favorite characters. I think it is because she WAS funny, successful, witty, and 'one of the guys.' I think Elaine could have been easily interchanged from a male or female role given most male characters too are written to be funny, successful, witty, etc...Regardless, I'm glad she was part of the club. Those of us unmarried and without kids didn't have a lot of representation in the 90s. Nearly every female character, even today, is eventually written off to get married and have kids - even when she's previously voiced no interest in either (think the Big Bang Theory).