Defense is all awareness, positioning, effort, and teamwork. None of that has to do with talent, unless you're talking about the ability to focus and maybe some of the players have ADD? I dunno...
You don't need superior individual defenders to play excellent team defense. You need a coach who's going to make sure everyone is aware of their assignment/goal on the defensive end, make sure they're in the right places more often than not, not slacking off and being lazy, and have everyone working as a cohesive unit. A defensive center would help, but it's not the answer if the environment isn't up to par. According the the results on the court, MDA either doesn't teach, preach, and demand solid defense from his teams....or they just don't listen to him. Either way, that's an ineffective coach in terms of championship basketball. If the Spurs, Lakers, Celtics, Mavs, and Bulls, all with their current rosters, played defense the way the Knicks do, they'd have weak records as well. So I don't see how it could be our bench, "the trade," chemistry, or players lacking physical ability to play good D...cause Carlos Delfino, Larry Sanders, and John Salmons are such amazing lock down defenders...right? No? Well shit....
Actually, when you examine the Bucks roster, the only legit "lock down" defender they have is Mbah a Moute, everyone else is average, below average, or terrible in man-to-man D. But yet they have the 5th best defense in the league and their coach is Scott Skiles...who is notorious for demanding that his teams play D...so...coincidence? We have average man-to-man defenders...we don't have the teamwork and schemes. When players aren't playing "together" and executing the schemes, who's job is it to hold everyone accountable and right the ship? And if he's incapable of doing that, what happens to him?
I won't even touch the offensive end.