You're over the top in thinking that DDG is to blame for United losing to Barca and Sevilla. Come on man. The team completely collapsed in two seasons ago when chasing CL. This year, DDG and team made every point count and it was good enough to finish 3rd but make up the points difference to get into position for CL. Over the weekend, he did fumble a shot a bit, but cleared it away from Vardy but he was offside to begin with so no harm done.
Everton was a freak goal, but then he saved United's bacon a couple times afterwards that helped them secure a point. That point made a difference and Everton also had some good chances to equalize or take the lead without the freak goal that was conceded.
Defense and goal keeping wasn't all that much of an issue for United. It was trying to score enough goals in the beginning few months and their goals for is significantly lagging behind the two teams ahead of them.
DDG deserves to have one more full year before his position is seriously, seriously challenged because the club will be at a crossroad and they will choose Henderson.
Henderson has to work his way to England number 1 without interruption and that's going to be at Sheffield United. If he achieves that goal, and then has a good showing at Euros, his first major England tournament, then he's on a high that will roll into being United's number 1. But he will not be without his mistakes and hype being England number 1 and the huge scrutiny when United could have an outfield of players on the brink of seriously challenging for the league.
From the Big Dane, to the abyss, to VDS, to DDG, to Henderson. Henderson has enormous gloves to fill after VDS and DDG. He'll need a lot of support and a lot of resolve just to begin his United career. And United have to have a reliable veteran goalkeeper.