You can't judge equally a young player and an experience CB.
Senderos has been part of CL final squad with tremendous stats. I consider his experience with us not great but not bad. If you remember, after us he played with one of the best team in the World: AC Milan.
Same words for Pascal Cygan for a different player. He was a fringe player and in 2005/06 season, with him on the pitch, Arsenal won 12 games on 20 games with 11 clean sheets. If you judge him like a horrible player to remember with our colours, I don't know why I'm talking with you.
You can't judge him like Silvestre (31 years old when he joined us, panic buy), Sebastian Squillaci (30 years, free agent).
If you want to talk about young defenders without experience since 2000 to 2016, I can mention Upson (good player for a midtable team), Ashley Cole (worldclass defender), Kolo Tourè (top class defender), Gael Clichy (very good defender), Justin Hoyte (shit), Senderos (good but not great), Djourou (shit), Ebouè (very good), Armand Traorè (shit), Kyele Bartley (shit), Ignasi Miquel (shit), Kieran Gibbs (good), Jenkinson (good), Bellerìn (top class), Chambers (good), Holding and Bielik (they are developing very good).... and I refused to mention the players who have developed under Arsène but they aren't too young like the others (Koscielny, Sagna, Vermaelen etc..)
Chamakh, Gervinho, Park and Campbell have been bought when van Persie was here and they were here because anyone scouted them. Especially Gervinho, has been an average player not because Wenger wasn't good, but because his qualities weren't adapt for a dynamic league like the Premier League. Chamakh has been useful, like Joel Campbell. The only big failure have been Park, but we don't know because he never played seriously with our shirt, and Yaya Sanogo.
...and apart Giroud, we bought Alexis Sanchez, Lukas Podolski,