I've always thought that if you give any PL goalkeeper ten shots to save per game they'll save most of them and look amazing. That's not the hallmark of a top goalkeeper for a big team. When we were at our worst defensively he was made to look better than he actually is.
It's not just his mistakes either, it's the timing of them. It was a series of his last season that cost us top 4, for example. World cup. Etc etc.
Add to that the fact that we regularly concede goals due to him being rooted to his line every cross, him being poor at playing with his feet at the back and poor communication with defenders and he's simply not good enough. The absolute minimum next season should be to have Henderson sat on the bench putting some real pressure on. Maybe that'll give him a kick up the arse.