For his size, there are few more talented players in the NBA (Garnett? Duncan?) and none of them can shoot long-range like Dirk can.
But lead a team? Typical Euro-junk on that point. Johnson saw that last season, and in particular in the playoffs against Miami. You can not now, or ever, play games of this caliber leaning on Nowiski as your floor leader, period.
So Johnson wanted him gone. A more reasonable alternative was given him by Cuban, which was to instead bring a leader to Dirk. For all his failings, being a floor leader is NOT one of Jason Kidd's. He can let Dirk be who he is and lead the team as Avery Johnson understands that concept.