Impel Down isn't even the meat of this arc. These guys are just what are standing in the way of whats to come: Luffy chasing Ace to Marine Ford. Do you think that they will be simply obliterating everything they come into contact with when/if they reach their goal? Plus, even right now with Blackbeard. I imagine Blackbeard alone wouldn't be able to take on Luffy/Croc/Jimbei, but he has his entire crew there as well. So its not like there are no challenges for them in Impel Down, and its not like this arc will have no challenges for Luffy's group.