Even if Auba does join Arsenal, they still haven't necessarily added. If Giroud goes out the door, then their transfer window will read: Auba & Mkhi - in, Sanchez & Giroud - out. Given how Sanchez has been their talisman the last 3/4 seasons, and how poor Mkhi has been recently, I'm not sure if that's as much an improvement as some seem to be suggesting. I think what's impressed most is that it isn't the total disaster some might have expected.
Also, Giroud will be a real miss for them. He offers something genuinely different upfront, and is a proven match-saver/winner in the EPL. Auba is clearly more talented, but if he requires any bedding in period then the short-term rationale behind this has to be questioned.