The floor opens up without DeRozan, giving everybody better looks. I don't think you need to replace DeRozan with a 20 ppg guy. A 12-15 ppg guy who plays D and shoots threes would make it much more difficult to double team Aldridge, and give White, Murray, Walker, Gay, etc., more space in the paint. So 9 more points total from the top 7 players combined plus 12 from his replacement = 21.
And if Klay replaced DeRozan, the Spurs would be drastically better... those assist & rebound numbers wouldn't mean anything.