Ya, he's probably going to suck, you're right. But it's not like Chris Rock, Bill Mayer or Steve Carell are gonna take over. Jimmy should have good improv w/ the guests, but I would not be surprised if the show cycles through hosts for a while like The late show on CBS did.
Nothing can nosedive faster and harder than Chevy Chase when FOX gave him a late night show in the 90s. I don't know if you guys were old enough to remember that fiasco. I think it lasted about a month. Magic Johnson did better with his own show.