Losing Thabo hurts but I don't think it's the difference between being a contender and not. The dropoff from Sefalosha to Kent Bazemore as a one-on-one defender is not that drastic. Carroll and Bazemore did a great job against LeBron last time they played Cleveland, for example.
The problem as I see it is that, unless Budenholzer is a genius who still has a hundred wrinkles up his sleeve that he hasn't had to show yet, I don't see how the Hawks can play 4-7 games in a row against a good team without the opponent adjusting to and neutralizing some of the Hawks' system, on both ends of the floor. And while the other contending teams have players who can get buckets even when the opponent defends them perfectly, the Hawks don't.
Maybe they can beat Cleveland, because the Cavaliers appear to be essentially uncoached, but I can't see them going up against the Warriors and Spurs for a whole series without the other team figuring them out.