Personally I think DDG is like Casillas, early peak and then pretty much done by 30. Henderson was one of the best in the league last season for Sheffield United, literally the only player they lost from their back line and they finished bottom with 30 more goals conceded. This United team with Henderson in goal look more comfortable at the back, let in less goals and has kept more clean sheets in half as many games than with DDG.
I think most people think DDG should still play because of the saves he could make. Forgetting that now his reactions have decreased, he offers very little in every other aspect of a top keeper. And let's be honest those top class reaction saves aside, he was never a sweeper keeper, commanding, vocal or great at passing out from the back.
You wouldn't keep playing a striker who could pull out a worldy of a goal every now and then to remind everyone that he's still got something, when you have a younger one who is scoring more goals and is better for the team overall. It's the same here with Henderson, he is a better fit for the team overall and how Ole wants to play.
I've posted this before, Schmeichel explains it better than I ever could.