The 76ers are still on the hook for a lot of McGee's salary after waiving him.
So DA can sign McGee on the cheap. 11-12m of cap space is very valuable, likely more valuable than picking up an extra 1st. So being patient to land McGee looked like the best move.
Ainge can have his cake and eat it too. He gets a low cost, high talent big man, and still has cap flexibility for free agency in 2016.