Yeah, Mertesacker is still so underrated here. He is the only defender that made Koscielny play at his highest level. Mert + Kos partnership is one of the best under Wenger, probably only behind Adams/Keown and Campbell/Toure.
Chambers was just played in the wrong position at the start. Even in the first season whenever he played CB he looked great. There's a good defender there in the making, I would say 16m for him will look like a bargain in the near future.
Gabriel for 10m was a really smart buy too. He was fantastic in a few big matches he played for us, and is underrated in many others due to his "messy" image. 10m is really nothing for a defender of his age and we made a profit shortly after. Now he is one of the best defenders in La Liga again and indispensable for Valencia. We should have sold Mustafi, not him.