Most players are on a downward spiral after the age of 30. David Silva is probably capable of playing at premiership level until 35-38 because of the way he plays. He doesn't rely on speed, its just technical ability and vision.
I agree that Yaya's replacement is the crocodile nearest the canoe at the moment. Although he has been brilliant for us, he won't have the longevity Silva will bring. However, Couthino is definitely not the answer.
We need to be bringing someone like Lopez back. He is a natural successor IMO.