Teams at the top are the most difficult. Executives have complex, long-term challenges, heavy demands on their time, and they got where they are by being John Waynes. There's no need to throw out the hierarchy. Teams are the best way to integrate across structural boundaries. They are the best way to design and energize core processes.
Teams permit performance and learning at the same time.
There is no better way to become a learning organization than to have a team-based structure which thrives on people learning from peers. The learning endures.