Well the argument is, is the personnel different enough. And will the fucking meltdowns stop.
Even just... cutting out the meltdowns, we could get back to a point where we have something like the ~50th defense in the country. I think that's where we were when looking at the non-meltdown skewed first 30 minutes. Good enough for a FF, not good enough for a title, probably. Then you hope that the addition of Winslow and Okafor over who they are replacing on the defensive end, even if just by virtue of rebounding and size, as well as the evolution of Sheed/Cook/Amile's game, is enough to get it close to top 20. Probably not, but I could see in theory how it could play out that way, even with the same system.
Will Okafor be a better defender than when Amile was at center? probably not from a rotational point of view. Will he be able to rebound as well as Amile though, on the defensive end?
Will Okafor be a better defender than when Jabari was at center? It can't be any worse. Rebounding... I dunno.
Will Amile be a better defender than when Jabari was at PF? I would think so. Average to better rebounding.
Will Justise be a better defender and rebounder than Hood. I think we can assume yes, given how awful a rebounder Hood was.
Sheed vs himself a year older. Hopefully he doesn't regress like Quinn.
Cook/Tyus - who the hell knows. I'll say this - Cook and Rasheed were once soph and freshmen starters on a defense that was top 5. Obviously that was because of Ryan, but the point is in theory, there mere presence alone doesn't preclude a defense from being elite.
I would hope... just by virtue of size and thus defensive rebounding that the defense is better, assuming the fucking awful mental meltdowns stop.
The truth is probably in between the knee-jerk "nothing is going to change" reaction and the "everything will work out ideally" reaction.