Well, formar SNL hacks like Adam Sandler, David Spade, and Chris Farley have/had made a career out of playing the exact same character 90% of the time also. I don't how they did it or why people flock to go see it but Fallon will somehow manage to get another job something, unfortunantly.
With the exception of Will Farrell and possible Tina Fey, SNL has been a wasteland for at least the past 10 years. I suffered through 3 episodes of it this season and it was just so lackluster.
Jimmy Fallon gave a pretty good performance in the film "Almost Famous". A lot of folks did not even realize it was him. I kinda doubt he is interested in making a bunch of films in the Adam Sandler/Rob Schneider mold, but I guess we'll see.
I think him leaving would be a good thing. I am tired of his smirks and gigles at his own jokes. When you are that pleased with yourself it's time to go and have your ego deflated by Hollywood. I thought the first half of the show was really funny. They had some good stuff written for The Olsens.