Atletico wasn't a threat in the league at that time, and now they are making real challenge in the league and CL, and they did won the league once after Simeone joined.
This is just a different ball game now when they can actually win something.
Okay, let's say Wenger does activate his release clause, it is still up to the player to sign or not.
Griezmann has the ability to join any top club in this world, and Wenger is not going to pay him outrageous salary to lure him here (and upset both Ozil and Sanchez).
In reality, this is not going to happen, for both seller and buyer.
It is not always about money.