Well, I suppose it's possible that Norm leaves Update without leaving the show entirely. But aside from Dan Aykroyd, I can't think of anyone who left Update while remaining on the show. Chase left update when he left the show, same with Jane Curtin and Bill Murray if I remember the facts correctly. Granted my knowledge of the show between the time the original cast and writers leave and Michaels' second season after returning is fairly limited. Basically, I know Eddie Murphy was on it and that's about it. So maybe there's precedent in those seasons, not sure. But in the post Miller era I think that was definitely the tradition. Miller left Update when he left the show, same with Nealon, Quinn, Fallon, and Fey. So I'm not sure such a departure is inevitable.
Speaking of Chevy Chase, I just thought of something else. What happens if Ford wins in 1976? As far as I know Chase decision to leave the show had been finalized before the election. So, with Chase gone and Ford still in the White House, what does SNL do? My guess is that the role would turn to Aykroyd. Well either that or no President Ford impressions at all, aside from when Chase hosts, for the remainder of the first five seasons. This may be a bit like what happened when Ferrell left. Other actors did Bush impressions in his absence. But when one tries to recall "SNL's George W. Bush" it's Ferrell that comes to mind-not Will Forte.