Jon Stewart cannot make the leap to anchoring a daily hard news show on a national network until he's put a few yrs distance between himself and that comedy show. he'd need to work as a journalist first.
if he gets the NBC Nightly News job i'd expect several NBC actual journalists would be outraged that they'd been jumped by a comedian w/no news experience. and actual journalists at other outlets would have a field day covering the story of his hiring. NBC News would be a joke - literally and figuratively. inside and out.