Nobody can replace Dave, but Colbert is genuinely funny. The thing is, he made his career off of being a character and it seems, no matter what he does, he's acting instead of just going with the flow (what Dave did best). I'm sure he can improvise during an interview, but what remains to be seen is if he can just be himself for an hour. Say what you want about Fallon, but dude is himself all the time and incredibly relatable to the audience. Same with Kimmel.