I agree that our success will start with defense.
I have a hard time criticizing Holiday for not being able to stay in front of SGA. What defender in the NBA can keep the quick, elite-scoring guards in front of them *consistently*? Answer: nobody. Without hand-checking and physicality, it can't happen. No defender can "out quick" SGA.
I think Tatum would struggle as the primary cover for him. Brown is the guy I would try. He's got a big size and strength advantage, while still have good (not elite) quickness and defensive smarts.
Another part of the problem is that, outside of KP, we have nobody that can protect the rim. Between Al, Kornet, Queta, and Tillman, not a one of them really deters drives to the basket. And, KP's rim protection hasn't seemed as effective this year as last (that's completely anecdotal on my part; I haven't looked up stats).
---------------------------------------------------------------------
I'm getting frustrated that the default offense is "chuck threes". There's a philosophy of "if it's a good shot, take it, no matter the circumstances". However, 1) a lot of the threes we're taking seem rushed and aren't "good shots"; and 2) sometimes your team just needs a bucket, and there is a much higher percentage of scoring inside than there is from outside.