Doug has failed in a number of ways this season, but Carson bears some responsibility for his play, both in his some of his terrible on field decisions, and in how he prepares. There's no excuse for a 5th year vet to have footwork as sloppy as his is. If you have replays available, watch Carson's footwork as he throws in comparison to *any* competent vet QB. Every offseason you read stories about how some player is working with some external coach to hone their technique in some way. It's clear that Wentz is doing nothing when in comes to improving his performance in the technicalities of playing QB. Hopefully this season has been a reality check for him and he focuses on improvement as a player rather than scheme and play calling.
If he does, I believe he can be a top 10 QB again, but it's mostly up to him.
Edit to add: I certainly wouldn't object if Doug were fired, as long as Howie is gone as well, but I don't think Doug is the biggest reason for Wentz's performance this season.