I know we are not getting Al Horford but what shouldn't get overlooked is Atlanta will very likely want to do a complete rebuild if he goes (they've already offloaded Teague). If that happens then Millsap becomes available and a trade for Leonard, Plumlee and Cleveland's pick or Vonleh might become attractive. It's not a perfect deal but not sure they are getting better for a player who is 31 and makes $22M a year.
Then if we don't sign a big in free agency (probable) and 76ers express interest in Crabbe, we need to remember they have a dispensable center. They couldn't even get a top 8 pick for Noel or Okafor which means they are probably not as valued across the league as they might have thought, maybe they'd endorse the possibility of getting Crabbe + Vonleh/Cleveland's pick (whatever's left from Millsap deal) for Noel?