This season could turn out to be similar to the 4th season of MASH, which was considered to be the worst, when the creative genius for that show, Larry Gelbart left and the series also had to endure McLean Stevenson and Wayne Rogers leaving. Of course that series ran for another 7 seasons but the emphasis changed more to the inter-personal relationships between the characters rather than the comedy itself.
If I had to bet, Seinfeld will run another season or two at most. It seems like Jerry, Julia, Jason and Michael are staying around more for the money due to the show being at its highest point in the ratings last season then it ever was. The latter 3, in particular, would grow a lot more as actors, if they moved on to other things. If the ratings drop significantly during the course of this season, (and they might given that the majority of posts so far did not like the first 2 episodes) - they'll call it quits.
If Seinfeld does have much more of a run - it will certainly be a different series from what it was during the first 7 seasons, perhaps even if Larry David agrees to return.