I cover this in the video I have going live tomorrow, but his replacement seems to be
Joe Piepiora, who seems to have been promoted internally from within the team. This strikes me as a positive sign - they did not "fire" David Kim and try to bring in an outside hire to replace him. Speculation, but with the Shadowlands delay being announced around the same time as David Kim being moved over to WoW, seems as though WoW really needed the help and perhaps Kim had laid enough groundwork in D4 for others to take over for him to finish the job.