I agree that GPS is becoming more popular and in the US, ADFs are becoming irrelevant. However, there are a lot of aircraft that aren't GPS equipped. Additionally, when terrain is a factor, what happens when you don't have RAIM? This becomes more of a problem for DME/DME equipped GPS aircraft over the move expensive INS/GPS.

If you look overseas, I don't see ADF going away anytime soon. Not in my lifetime. When you fly in South America and Africa, you literally go back 30 years in aviation. When you fly in the former soviet states, a lot of those routes are ADF to ADF.