IIRC Costa's transfer to the scousers was all but done but he refused the move because Liverpool had no CL football and he'd have been better off staying where he is.
I don't think he has any loyalty to Atletico. Doesn't strike me as the loyal type anyway. After all he turned his back on his country.
It's all about the money at the end of the day and how much success we can offer him. If we offer to triple his wages in January, would he say no? He's on very low wages apparently and we can easily offer him a significant improvement on his current salary.
Then there is the Wenger-pull factor. Players just want to play for him.
As for his World Cup spot, the man is a world class striker. He has a spot already sewn up whether he stays or leaves.