I love "Seinfeld"...THE SHOW. I have all the seasons on DVD. But "Seinfeld" was really a "period" sitcom -- very 1990's. When I see episodes, there are a LOT of lines that simply aren't relevant in the 21st century, and some that border on offensive in the post-9/11 world. For example, in one episode he jokes about firemen, "Yeah those first responders who risk their lives really have it made", dripping with sarcasm. Or all the innuendo about sex, like "The Contest"; sitcoms today regularly say masturbate. I could go on. Additionally, I daresay people watched "Seinfeld" to see Jerry the character, not Jerry the stand-up comic. The show certainly never made me want to see Jerry doing stand-up, even on TV. People will initially tune to the commericals expecting to see Jerry the character, or a "Seinfeld" (the show) styled commerical, and will be disappointed to see Jerry the (mediocre) comedian. Finally, "Seinfeld" was successful because of the sum of its other parts -- Larry David, Jason Alexander, Michael Richards, and Julia Louise-Dreyfus.
Much as I still love "Seinfeld", Jerry himself is not really a relevant or hip celebrity. Apple did the same thing once, when they used Alan Alda to promote Macs. Once sitcoms end, they usually pass from the public's mind very quickly, with some exceptions. (It might be funnier if someone used George and Weezy Jefferson.) If Microsoft really wanted to appeal to a younger base, they should have used "Family Guy" or "The Simpsons", or John Stewart. (Or hire Wayne Knight (Newman) to appear with Jerry...then it might be interesting to watch!)
But in the end, does it really matter who the spokesperson is? I mean really...has anyone really gone out and bought a PC because HP has Gwen Stefani or Jay-Z in its commercials? I would doubt it.