there are 2groups of players that Wenger has let go of in recent times:
1-Players that are past their primes and or on the decline: Vieira, Pires, Henry..etc. Wenger didn't just decide to let them leave, and hey didn't just suddenly turn bad the moment they left Arsenal, they were already on the decline in their last season at Arsenal. Wenger saw that and he sold them (especially since he had their replacements ready)
2- players that were not good enough for Arsenal: Bentely, Cygan, and the likes.
if for example we get someone like Fabregas or Toure or Gallas, then it's not like they are going to play like crap for us, and it is the reason why Wenger would never try to sell them yet. Because these players will be stars wherever they go. But watch the moment that that they reach towards their 30s and they have a bad season then he will sell them