Lorenzo was the better player and has proven to be a team leader which McD clearly values. He is older but has less wear since he hasn't been a full time starter all these years. We get two good years from him while he mentors the new guys. That's the idea.
I'm also not in the "we are tanking" camp, and I think this is one of the pieces of evidence. The leaders are staying on despite their age, because they want guys that will go play their hardest while serving as mentors. McD is also looking for younger guys to be the new leaders. Micah Hyde is one guy who is positioning himself in that role.